5.1. ALM: Input files

Format of input files

Each input file should consist of entry fields. Available entry fields are

&general, &interaction, &cutoff, &cell, &position, and &optimize.

Each entry field starts from the key label &field and ends at the terminate character “/”. (This is equivalent to Fortran namelist.)

For example, &general entry field of program alm should be given like

&general
  # Comment line
  PREFIX = prefix
  MODE = optimize
/

Multiple entries can be put in a single line. Also, characters put on the right of sharp (“#”) are neglected. Therefore, the above example is equivalent to the following:

&general
  PREFIX = prefix; MODE = optimize  # Comment line
/

Each variable must be given inside the appropriate entry field.

List of supported input variables

&general
HESSIAN	FC3_SHENGBTE	FCSYM_BASIS	FC_ZERO_THR
KD	MAGMOM	MODE	NAT
NKD	NMAXSAVE	NONCOLLINEAR	PERIODIC
PREFIX	PRINTSYM	TOLERANCE
&interaction
NBODY	NORDER
&optimize
CONV_TOL	CV	CV_MINALPHA	DEBIAS_OLS
DFSET	DFSET_CV	ENET_DNORM
FC2XML	FC3XML	ICONST
L1_ALPHA	L1_RATIO	LMODEL	MAXITER
NDATA	NDATA_CV	NSTART NEND	NSTART_CV NEND_CV
PERIODIC_IMAGE_CONV	ROTAXIS	SKIP
SOLUTION_PATH	SPARSE
SPARSESOLVER	STANDARDIZE	STOP_CRITERION

Description of input variables

“&general”-field

PREFIX-tag : Job prefix to be used for names of output files

Default:

None

Type:

String

MODE-tag = optimize | suggest

optimize (>= 1.1.0)

Estimate harmonic and anharmonic IFCs.

This mode requires an appropriate &optimize field.

suggest

Suggests the displacement patterns necessary

to estimate harmonic and anharmonic IFCS.

Default:

None

Type:

String

NAT-tag : Number of atoms in the supercell

Default:

None

Type:

Integer

NKD-tag : Number of atomic species

Default:

None

Type:

Integer

KD-tag = Name[1], … , Name[NKD]

Default:

None

Type:

Array of strings

Example:

In the case of GaAs with NKD = 2, it should be KD = Ga As.

TOLERANCE-tag : Tolerance for finding symmetry operations

Default:

1.0e-3

Type:

Double

PRINTSYM-tag = 0 | 1

0

Symmetry operations won’t be saved in “SYMM_INFO”

1

Symmetry operations will be saved in “SYMM_INFO”

Default:

0

type:

Integer

FCSYM_BASIS-tag = Cartesian | Lattice

Cartesian, C

Symmetry reduction of force constant is performed in the Cartesian basis

Lattice, L

Symmetry reduction of force constant is performed in the \(\boldsymbol{a}_1, \boldsymbol{a}_2, \boldsymbol{a}_3\) basis

Default:

Lattice

type:

String

Description:

The calculation results should not depend on the choice of FCSYM_BASIS when LMODEL = ols. For other regression methods (enet, adaptive LASSO), an optimal value of the L1_ALPHA changes when you change the FCSYM_BASIS option.

In some cases, FCSYM_BASIS = Lattice is more stable and efficient. In particular, we recommend setting FCSYM_BASIS = Lattice for hexagonal systems. If a calculation with FCSYM_BASIS = Lattice is slow, please switch to FCSYM_BASIS = Cartesian.

For more details about the symmetry reduction of force constants, please see here.

Important

When FCSYM_BASIS = Lattice, the basis of force constants saved in PREFIX.fcs becomes the \(\boldsymbol{a}_1, \boldsymbol{a}_2, \boldsymbol{a}_3\) basis. Hence, to compare the values of force constants saved in PREFIX.fcs, you will have to change their basis to the Cartesian basis manually. The basis of force constants saved in PREFIX.xml is Cartesian irrespective of the FCSYM_BASIS value.

Also, imposing the constraints for rotational invariance with FCSYM_BASIS = Lattice is not supported. Therefore, if you want to apply the constraints for rotational invariance, please use FCSYM_BASIS = Cartesian.

MAGMOM-tag : List of magnetic moments

Default:

0 … 0 (NAT entries when NONCOLLINEAR = 0, 3xNAT entries when NONCOLLINEAR = 1.)

type:

Array of double

Example:

When a supercell containts 64 atoms and the local magnetic moments of the first 32 atoms are up and those of the last 32 atoms are down, please set the MAGMOM tag as MAGMOM = 32*1 32*-1. The wildcard (*) is available when NONCOLLINEAR = 0. For the noncollinear case (NONCOLLINEAR = 1), the wildcard is not supported. So, please give the magnetic moment explicitly as MAGMOM = 0 0 1 0 0 1 0 0 1 ... 0 0 -1 0 0 -1 ... (3\(\times\)NAT entries in one line).

Note

MAGMOM information is used only for generating space group operations. So, the values of the magnetic moment are somewhat arbitrary. For the 4\(\times\) 4\(\times\) 4 supercell of ferromagnetic bcc Fe (64 atoms), for instance, MAGMOM = 64*1 and MAGMOM = 64*2 give the same results. By contrast, MAGMOM = 32*1 32*2 of course gives a different result because it breaks the symmetry of the original lattice.

NONCOLLINEAR-tag = 0 | 1

Default:

0

type:

Integer

Description:

When NONCOLLINEAR = 1, the code accepts a noncollinear magnetic structure as an input to the MAGMOM tag and uses it for generating space group operations. The spin quantization axis is fixed to the (0,0,1) direction of the Cartesian axis.

Caution

Still experimental. Please use with care.

PERIODIC-tag = PERIODIC[1], PERIODIC[2], PERIODIC[3]

0

Do not consider periodic boundary conditions when

searching for interacting atoms.

1

Consider periodic boundary conditions when

searching for interacting atoms.

Default:

1 1 1

type:

Array of integers

Description:

This tag is useful for generating interacting atoms in low dimensional systems. When PERIODIC[i] is zero, periodic boundary condition is turned off along the direction of the lattice vector \(\boldsymbol{a}_{i}\).

NMAXSAVE-tag : The maximum order of anharmonic force constants printed out in PREFIX.xml

Default:

min(5, NORDER)

Type:

Integer

Example:

If your model includes anharmonic terms up to the sixth-order (NORDER = 5), but you want to avoid printing out the fifth-order and sixth-order IFCs in PREFIX.xml, please set NMAXSAVE = 3.

HESSIAN-tag = 0 | 1

0

Do not save the Hessian matrix

1

Save the entire Hessian matrix of the supercell as PREFIX.hessian.

Default:

0

type:

Integer

FC3_SHENGBTE-tag = 0 | 1

0

Do not save the third-order force constants for ShengBTE code

1

Save the third-order force constants for the ShengBTE code in PREFIX.FORCE_CONSTANT_3RD.

Default:

0

type:

Integer

FC_ZERO_THR-tag : Threshold value used when trimming force constants when creating PREFIX.xml

Default:

1.0e-12

Type:

Double

Description:

FC_ZERO_THR defines the threshold of force constants to be saved in an XML file. If the absolute value of force constant is smaller than FC_ZERO_THR, it will NOT be printed out.

Note

If the harmonic force constants are calculated using a model potential (e.g., classical FF) where the interaction becomes zero beyond a certain cutoff raius, the default value of FC_ZERO_THR may raise a warning when creating a renormalize harmonic FCSXML using tools/dfc2. This issue may be resolved by using a smaller FC_ZERO_THR, say FC_ZERO_THR = 1.0e-15. The force constants that become exactly zero due to symmetry and acoustic sum rule constraints will not be printed even when setting FC_ZERO_THR = 0.

“&interaction”-field

NORDER-tag : The order of force constants to be calculated. Anharmonic terms up to \((m+1)\)th order will be considered with NORDER = \(m\).

Default:

None

Type:

Integer

Example:

NORDER = 1 for calculate harmonic terms only, NORDER = 2 to include cubic terms as well, and so on.

NBODY-tag : Entry for excluding multiple-body interactions from anharmonic force constants

Default:

NBODY = [2, 3, 4, …, NORDER + 1]

Type:

Array of integers

Description:

This tag may be useful for excluding multi-body clusters which are supposedly less important. For example, a set of fourth-order IFCs \(\{\Phi_{ijkl}\}\), where \(i, j, k\), and \(l\) label atoms in the supercell, can be categorized into four different subsets; on-site, two-body, three-body, and four-body terms. Neglecting the Cartesian coordinates of IFCs for simplicity, each subset contains the IFC elements shown as follows:

on-site

\(\{\Phi_{iiii}\}\)

two-body

\(\{\Phi_{iijj}\}\), \(\{\Phi_{iiij}\}\) (\(i\neq j\))

three-body

\(\{\Phi_{iijk}\}\) (\(i\neq j, i\neq k, j \neq k\))

four-body

\(\{\Phi_{ijkl}\}\) (all subscripts are different from each other)

Since the four-body clusters are expected to be less important than the three-body and less-body clusters, you may want to exclude the four-body terms from the Taylor expansion potential because the number of such terms is huge. This can be done by setting the NBODY tag as NBODY = 2 3 3 together with NORDER = 3.

More examples:

NORDER = 2; NBODY = 2 2 includes harmonic and cubic IFCs but excludes three-body clusters from the cubic terms.

NORDER = 5; NBODY = 2 3 3 2 2 includes anharmonic terms up to the sixth-order, where the four-body clusters are excluded from the fourth-order IFCs, and the multi (\(\geq 3\))-body clusters are excluded from the fifth- and sixth-order IFCs.

“&cutoff”-field

In this entry field, one needs to specify cutoff radii of interaction for each order in units of bohr. In the current implementation, cutoff radii should be defined for every possible pair of atomic elements. For example, the cutoff entry for a harmonic calculation (NORDER = 1) of Si (NKD = 1) should be like

&cutoff
 Si-Si 10.0
/

This means that the cutoff radius of 10 \(a_{0}\) is used for harmonic Si-Si terms. Please note that the first column should be two character strings, which are contained in the KD-tag, connected by a hyphen (’-’).

When one wants to consider cubic terms (NORDER = 2), please specify the cutoff radius for cubic terms in the third column as the following:

&cutoff
 Si-Si 10.0 5.6 # Pair r_{2} r_{3}
/

Instead of giving specific cutoff radii, one can write “None” as follows:

&cutoff
 Si-Si None 5.6
/

which means that all possible harmonic terms between Si-Si atoms will be included.

Caution

Setting ‘None’ for anharmonic terms can greatly increase the number of parameters and thereby increase the computational cost.

When there are more than two atomic elements, please specify the cutoff radii between every possible pair of atomic elements. In the case of MgO (NKD = 2), the cutoff entry should be like

&cutoff
 Mg-Mg 8.0
 O-O 8.0
 Mg-O 10.0
/

which can equivalently be written by using the wild card (’*’) as

&cutoff
 *-* 8.0
 Mg-O 10.0 # Overwrite the cutoff radius for Mg-O harmonic interactions
/

Important

Cutoff radii specified by an earlier entry are overwritten by a new entry that comes later.

Once the cutoff radii are properly given, harmonic force constants \(\Phi_{i,j}^{\mu,\nu}\) satisfying \(r_{ij} \le r_{c}^{\mathrm{KD}[i]-\mathrm{KD}[j]}\) will be searched.

In the case of cubic terms, force constants \(\Phi_{ijk}^{\mu\nu\lambda}\) satisfying \(r_{ij} \le r_{c}^{\mathrm{KD}[i]-\mathrm{KD}[j]}\), \(r_{ik} \le r_{c}^{\mathrm{KD}[i]-\mathrm{KD}[k]}\), and \(r_{jk} \le r_{c}^{\mathrm{KD}[j]-\mathrm{KD}[k]}\) will be searched and determined by fitting.

“&cell”-field

Please give the cell parameters in this entry in units of bohr as the following:

&cell
 a
 a11 a12 a13
 a21 a22 a23
 a31 a32 a33
/

The cell parameters are then given by \(\vec{a}_{1} = a \times (a_{11}, a_{12}, a_{13})\), \(\vec{a}_{2} = a \times (a_{21}, a_{22}, a_{23})\), and \(\vec{a}_{3} = a \times (a_{31}, a_{32}, a_{33})\).

“&position”-field

In this field, one needs to specify the atomic element and fractional coordinate of atoms in the supercell. Each line should be

ikd xf[1] xf[2] xf[3]

where ikd is an integer specifying the atomic element (ikd = 1, …, NKD) and xf[i] is the fractional coordinate of an atom. There should be NAT such lines in the &position entry field.

“&optimize”-field

This field is necessary when MODE = optimize.

LMODEL-tag : Choice of the linear model used for estimating force constants

“least-squares”, “LS”, “OLS”, 1

Ordinary least square

“elastic-net”, “enet”, 2

Elastic net

“adaptive-lasso”, 3

Adaptive LASSO

Default:

least-squares

Type:

String

Description:

When LMODEL = ols, the force constants are estimated from the displacement-force datasets via the ordinary least-squares (OLS), which is usually sufficient to calculate harmonic and third-order force constants.

The elastic net (LMODEL = enet) or adaptive LASSO (LMODEL = adaptive-lasso) are useful for calculating fourth-order (and higher-order) force constants. When the elastic net or adaptive LASSO is selected, the users have to set the following related tags: CV, L1_RATIO, L1_ALPHA, CV_MAXALPHA, CV_MINALPHA, CV_NALPHA, STANDARDIZE, ENET_DNORM, MAXITER, CONV_TOL, NWRITE, SOLUTION_PATH, DEBIAS_OLS, STOP_CRITERION. Please be noted that STANDARDIZE will be effective only for the elastic net.

DFSET-tag: File name containing displacement-force datasets for training

New in version 1.1.0.

Default:

None

Type:

String

Description:

The format of DFSET can be found here

NDATA-tag : Number of displacement-force data sets

Default:

None

Type:

Integer

Description:

If NDATA is not given, the code reads all lines of DFSET (excluding comment lines) and estimates NDATA by dividing the line number by NAT. If the number of lines is not divisible by NAT, an error is raised. DFSET should contain at least NDATA\(\times\) NAT lines.

NSTART, NEND-tags : Specifies the range of data to be used for training

Default:

NSTART = 1, NEND = NDATA

Type:

Integer

Example:

To use the data in the range of [20:30] out of 50 entries, the tags should be NSTART = 20 and NEND = 30.

SKIP-tag : Specifies the range of data to be skipped for training

Default:

None

Type:

Two integers connected by a hyphen

Description:

SKIP =\(i\)-\(j\) skips the data in the range of [\(i\):\(j\)]. The \(i\) and \(j\) must satisfy \(1\leq i \leq j \leq\) NDATA. This option may be useful when doing cross-validation manually (CV=-1).

ICONST-tag = 0 | 1 | 2 | 3 | 11

0

No constraints

1

Constraint for translational invariance is imposed between IFCs.

Available only when LMODEL = ols.

11

Same as ICONST = 1 but the constraint is imposed algebraically rather than numerically.

Select this option when LMODEL = enet.

2

In addition to ICONST = 1, constraints for rotational invariance will be

imposed up to (NORDER + 1)th order. Available only when LMODEL = ols.

3

In addition to ICONST = 2, constraints for rotational invariance between (NORDER + 1)th order

and (NORDER + 2)th order, which are zero, will be considered.

Available only when LMODEL = ols.

Default:

11

Type:

Integer

Description:

See this page for the numerical formulae.

PERIODIC_IMAGE_CONV-tag = 0 | 1

0

Impose the constraints on IFCs (acoustic sum rule) in the considering supercell.

1

Consider the periodic images when generating the constraints.

The resultant IFCs simultaneously satisfy the permutation symmetry, ASR,

and the space group symmetry in the infinite space.

For more details, please see Appendix D of the original paper.

(Note that we use the term “mirror image” instead of “periodic image” in the paper.)

Default:

1

Type:

Integer

ROTAXIS-tag : Rotation axis used to estimate constraints for rotational invariance. This entry is necessary when ICONST = 2, 3.

Default:

None

Type:

String

Example:

When one wants to consider the rotational invariance around the \(x\)-axis, one should give ROTAXIS = x. If one needs additional constraints for the rotation around the \(y\)-axis, ROTAXIS should be ROTAXIS = xy.

FC2XML-tag : XML file to which the harmonic terms are fixed upon training

Default:

None

Type:

String

Description:

When FC2XML-tag is given, harmonic force constants are fixed to the values stored in the FC2XML file. This may be useful for optimizing cubic and higher-order terms without changing the harmonic terms. Please make sure that the number of harmonic terms in the new computational condition is the same as that in the FC2XML file.

Important

The FCSYM_BASIS option must be the same as the one used when creating the reference harmonic force constant file (FC2XML). The code raises an error when they are inconsistent.

FC3XML-tag : XML file to which the cubic terms are fixed upon training

Default:

None

Type:

String

Description:

Same as the FC2XML-tag, but FC3XML is to fix cubic force constants.

Important

The FCSYM_BASIS option must be the same as the one used when creating the reference cubic force constant file (FC3XML). The code raises an error when they are inconsistent.

SPARSE-tag = 0 | 1

0

Use a direct solver (SVD or QRD) to estimate force constants

1

Use a sparse solver to estimate force constants

Default:

0

Type:

Integer

Description:

When you want to calculate force constants of a large system and generate training datasets by displacing only a few atoms from equilibrium positions, the resulting sensing matrix becomes large but sparse. For such matrices, a sparse solver is expected to be more efficient than SVD or QRD in terms of both memory usage and computational time. When SPARSE = 1 is set, the code uses a sparse solver implemented in Eigen3 library. You can change the solver type via SPARSESOLVER. Effective when LMODEL = ols.

SPARSESOLVER-tag : Type of the sparse solver to use

Default:

SimplicialLDLT

Type:

String

Description:

Currently, only the sparse solvers of Eigen3 library can be used. Available options are SimplicialLDLT, SparseQR, ConjugateGradient, LeastSquaresConjugateGradient, and BiCGSTAB. When an iterative algorithm (conjugate gradient) is selected, a stopping criterion can be specified by the CONV_TOL and MAXITER tags. Effective when LMODEL = ols and SPARSE = 1.

See also

Eigen documentation page: Solving Sparse Linear Systems

MAXITER-tag : Number of maximum iterations in iterative algorithms

Default:

10,000

Type:

Integer

Description:

Effective when an iterative solver is selected via SPARSESOLVER (when LMODEL = ols) or when LMODEL = enet | adaptive-lasso.

CONV_TOL-tag : Convergence criterion of iterative algorithms

Default:

1.0e-8

Type:

Double

Description:

When LMODEL = ols and an iterative solver is selected via SPARSESOLVER, CONV_TOL value is passed to the Eigen3 function via setTolerance(). When LMODEL = enet | adaptive-lasso, the coordinate descent iteration stops at \(i\)th iteration if \(\sqrt{\frac{1}{N}|\boldsymbol{\Phi}_{i} - \boldsymbol{\Phi}_{i-1}|_{2}^{2}} <\) CONV_TOL is satisfied, where \(N\) is the length of the vector \(\boldsymbol{\Phi}\).

See also

Eigen documentation page: IterativeSolverBase

L1_RATIO-tag : The ratio of the L1 regularization term

Default:

1.0 (LASSO)

Type:

Double

Description:

The L1_RATIO changes the regularization term as L1_ALPHA \(\times\) [L1_RATIO \(|\boldsymbol{\Phi}|_{1}\) + \(\frac{1}{2}\) (1-L1_RATIO) \(|\boldsymbol{\Phi}|_{2}^{2}\)]. Therefore, L1_RATIO = 1 corresponds to LASSO. L1_RATIO must be 0 < L1_ratio <= 1. Effective when LMODEL = enet. See also here.

L1_ALPHA-tag : The coefficient of the L1 regularization term

Default:

0.0

Type:

Double

Description:

This tag is used when LMODEL = enet | adaptive-lasso and CV = 0. See also here.

CV-tag : Cross-validation mode for elastic net

0

Cross-validation mode is off.

The elastic net optimization is solved with the given L1_ALPHA value.

The force constants are written to PREFIX.fcs and PREFIX.xml.

>= 2

CV-fold cross-validation is performed automatically.

NDATA training datasets are divided into CV subsets, and CV different combinations of

training-validation datasets are created internally. For each combination, the elastic net

optimization is solved with the various L1_ALPHA values defined by the CV_MINALPHA,

CV_MAXALPHA, and CV_NALPHA tags. The result of each cross-validation is stored in

PREFIX.cvset[1, …, CV], and their average and deviation are stored in PREFIX.cvscore.

-1

The cross-validation is performed manually.

The Taylor expansion potential is trained by using the training datasets in DFSET, and

the validation score is calculated by using the data in DFSET_CV for various L1_ALPHA values

defined the CV_MINALPHA, CV_MAXALPHA, and CV_NALPHA tags.

After the calculation, the fitting and validation errors are stored in PREFIX.cvset.

This option may be convenient for a large-scale problem since multiple optimization tasks with

different training-validation datasets can be done in parallel.

Default:

0

Type:

Integer

Description:

This tag is used when LMODEL = enet | adaptive-lasso.

DFSET_CV-tag : File name containing displacement-force datasets used for manual cross-validation

Default:

DFSET_CV = DFSET

Type:

String

Description:

This tag is used when LMODEL = enet | adaptive-lasso and CV = -1.

NDATA_CV-tag : Number of displacement-force validation datasets

Default:

None

Type:

Integer

Description:

This tag is used when LMODEL = enet | adaptive-lasso and CV = -1.

NSTART_CV, NEND_CV-tags : Specifies the range of data to be used for validation

Default:

NSTART_CV = 1, NEND_CV = NDATA_CV

Type:

Integer

Example:

This tag is used when LMODEL = enet | adaptive-lasso and CV = -1.

CV_MINALPHA, CV_MAXALPHA, CV_NALPHA-tags : Options to specify the L1_ALPHA values used in cross-validation

Default:

CV_MAXALPHA is set automatically

CV_MINALPHA = CV_MAXALPHA * 1.0e-6

CV_NALPHA = 50

Type:

Double, Double, Integer

Description:

CV_NALPHA values of L1_ALPHA are generated from CV_MINALPHA to CV_MAXALPHA in logarithmic scale. When CV_MAXALPHA is not specified by user, the code automatically sets CV_MAXALPHA so that the maximum L1_ALPHA makes all coefficients zero. The default value of CV_MINALPHA is CV_MAXALPHA * 1.0e-6, which is reasonable in many cases. If the minimum value of the validation score is found at CV_MINALPHA, you may need to use a smaller value of CV_MINALPHA. This tag is used when LMODEL = enet | adaptive-lasso and the cross-validation mode is on (CV > 0 or CV = -1).

STANDARDIZE-tag = 0 | 1

0

Do not standardize the sensing matrix

1

Each column of the sensing matrix is standardized in such a way that its mean value

becomes 0 and standard deviation becomes 1.

Default:

1

Type:

Integer

Description:

This option influences the optimal L1_ALPHA value. So, if you change the STANDARDIZE option, you have to rerun the cross-validation. Effective only when LMODEL = enet.

ENET_DNORM-tag : Normalization factor of atomic displacements

Default:

1.0

Type:

Double

Description:

The normalization factor of atomic displacement \(u_{0}\) in units of bohr. When \(u_{0} (\neq 1)\) is given, the displacement data are scaled as \(u_{i} \rightarrow u_{i}/u_{0}\) before constructing the sensing matrix. This option influences the optimal L1_ALPHA value. So, if you change the ENET_DNORM value, you will have to rerun the cross-validation. Effective only when LMODEL = enet and STANDARDIZE = 0.

SOLUTION_PATH-tag = 0 | 1

0

Do not save the solution path.

1

Save the solution path of each cross-validation combination in PREFIX.solution_path.

Default:

0

Type:

Integer

Description:

Effective when LMODEL = enet | adaptive-lasso and the cross-validation mode is on.

DEBIAS_OLS-tag = 0 | 1

0

Save the solution of the elastic net problem to PREFIX.fcs and PREFIX.xml.

1

After the solution of the elastic net optimization problem is obtained,

only non-zero coefficients are collected, and the ordinary least-squares fitting is

solved again with the non-zero coefficients before saving the results to PREFIX.fcs and

PREFIX.xml. This might be useful to reduce the bias of the elastic net solution.

Default:

0

Type:

Integer

Description:

Effective when LMODEL = enet and CV = 0.

STOP_CRITERION-tag : The scan over L1_ALPHA stops when the cross-validation score keeps increasing in STOP_CRITERION consecutive steps

Default:

5

Type:

Integer

Description:

Effective when LMODEL = enet | adaptive-lasso and the cross-validation mode is turned on (CV > 0 or CV = -1).

How to make a DFSET file

Format of `DFSET`

The displacement-force data sets obtained by first-principles (or classical force-field) calculations have to be saved to a file, say DFSET. Then, the force constants are estimated by setting DFSET = DFSET and with MODE = optimize.

The DFSET file must contain the atomic displacements and corresponding forces in Cartesian coordinate for at least NDATA structures (displacement patterns) in the following format:

# Structure number 1 (this is just a comment line) \begin{eqnarray*} u_{x}(1) & u_{y}(1) & u_{z}(1) & f_{x}(1) & f_{y}(1) & f_{z}(1) \\ u_{x}(2) & u_{y}(2) & u_{z}(2) & f_{x}(2) & f_{y}(2) & f_{z}(2) \\ & \vdots & & & \vdots & \\ u_{x}(\mathrm{NAT}) & u_{y}(\mathrm{NAT}) & u_{z}(\mathrm{NAT}) & f_{x}(\mathrm{NAT}) & f_{y}(\mathrm{NAT}) & f_{z}(\mathrm{NAT}) \end{eqnarray*} # Structure number 2 \begin{eqnarray*} u_{x}(1) & u_{y}(1) & u_{z}(1) & f_{x}(1) & f_{y}(1) & f_{z}(1) \\ & \vdots & & & \vdots & \end{eqnarray*}

Here, NAT is the number of atoms in the supercell. The unit of displacements and forces must be bohr and Ryd/bohr, respectively.

0	No constraints
1	Constraint for translational invariance is imposed between IFCs. Available only when `LMODEL = ols`.
11	Same as `ICONST = 1` but the constraint is imposed algebraically rather than numerically. Select this option when `LMODEL = enet`.
2	In addition to `ICONST = 1`, constraints for rotational invariance will be imposed up to (`NORDER` + 1)th order. Available only when `LMODEL = ols`.
3	In addition to `ICONST = 2`, constraints for rotational invariance between (`NORDER` + 1)th order and (`NORDER` + 2)th order, which are zero, will be considered. Available only when `LMODEL = ols`.

0	Cross-validation mode is off. The elastic net optimization is solved with the given `L1_ALPHA` value. The force constants are written to `PREFIX`.fcs and `PREFIX`.xml.
>= 2	`CV`-fold cross-validation is performed automatically. `NDATA` training datasets are divided into `CV` subsets, and `CV` different combinations of training-validation datasets are created internally. For each combination, the elastic net optimization is solved with the various `L1_ALPHA` values defined by the `CV_MINALPHA`, `CV_MAXALPHA`, and `CV_NALPHA` tags. The result of each cross-validation is stored in `PREFIX`.cvset[1, …, `CV`], and their average and deviation are stored in `PREFIX`.cvscore.
-1	The cross-validation is performed manually. The Taylor expansion potential is trained by using the training datasets in `DFSET`, and the validation score is calculated by using the data in `DFSET_CV` for various `L1_ALPHA` values defined the `CV_MINALPHA`, `CV_MAXALPHA`, and `CV_NALPHA` tags. After the calculation, the fitting and validation errors are stored in `PREFIX`.cvset. This option may be convenient for a large-scale problem since multiple optimization tasks with different training-validation datasets can be done in parallel.