MOLCAS manual:

Subsections

• 8.1.1 Analytic Gradients
• 8.1.2 Description
• 8.1.3 Numerical gradients
• 8.1.4 Dependencies
• 8.1.5 Files
  • 8.1.5.1 Input files
  • 8.1.5.2 Output files
  
  
• 8.1.6 Input

8.1 alaska

This module is automatically invoked by the Slapaf module. This is the preferred mode of operation! In connection with numerical gradients it will ensure that the rotational and translational invariance is fully utilized in order to reduce the number of used displacements.

The Alaska module compute analytic or for numerical gradients requests the execution of an alternative module. The Alaska module figures out the method automatically. Analytic methods are implemented for the HF, KS-DFT, and RASSCF and SA-CASSCF method. Numerical methods are implemented for SCF, KS-DFT, RASSCF, MBPT2, CCSDT, the CASPT2 and MS-CASPT2 methods, including the use of the Cholesky decomposition for the methods were that has been implemented.

Both analytic and numerical procedures are parallelized.

For SA-CASSCF gradient the Alaska module will automatically start up the MCLR module if required.

8.1.1 Analytic Gradients

Gradients of the energy with respect to nuclear coordinates can be computed for any type of wave function as long as an effective first order density matrix, an effective Fock matrix, and an effective second order density matrix is provided. The term effective is related to that these matrices in the case of non-variational parameters in the wave function (e.q. CI, MP2, CASPT2, etc.) are modified to include contributions from the associated Lagrange multipliers. The gradient expression apart from these modifications is the same for any wave function type. ALASKA is the gradient program, which will generate the necessary integral derivatives and combine them with the matrices mentioned in the text above.

8.1.2 Description

ALASKA is written such that gradients can be computed for any kind of basis function that SEWARD will accept.

ALASKA is able to compute the following integral derivatives:

• overlap integrals,
• kinetic energy integrals,
• nuclear attraction integrals (point charges or finite nuclei),
• electron repulsion integrals,
• external electric field integrals,
• ECP and PP integrals,
• reaction field integrals,
• and Pauli repulsion integrals.

ALASKA employs two different integration schemes to generate the one- and two-electron integral derivatives. The nuclear attraction and electron repulsion integrals are evaluated by a modified Rys-Gauss quadrature [82]. All other integral derivatives are evaluated with the Hermite-Gauss quadrature. The same restriction of the basis sets applies as to SEWARD. None of the integral derivatives are written to disk but rather combined immediately with the corresponding matrix from the wave function.

At present the following limitations are built into ALASKA:

Max number of unique basis functions:	2000
Max number of symmetry independent centers:	500
Highest angular momentum:	15
Highest symmetry point group:	D2h

8.1.3 Numerical gradients

The module is parallelized over the displacements, which in case of large jobs gives a linear speed up compared to a serial execution, although in order to obtain this it is important to choose the number of nodes such that the number of contributing perturbations is a multiple of the number of nodes. For a given molecule the number of perturbations equals the number of atoms times 6 (a perturbation with plus and minus delta for each of the three axises). Symmetry can of course reduce this number.

8.1.4 Dependencies

ALASKA depends on the density and Fock matrices generated by SCF or RASSCF. In addition it needs the basis set specification defined in SEWARD. The dependencies of the numerical part of the module is the union of the dependencies of the SEWARD, SCF, RASSCF, MBPT2, MOTRA, CCSDT, and CASPT2 modules. All these dependencies, however, are totally transparent to the user.

8.1.5 Files

8.1.5.1 Input files

Apart from the standard input unit ALASKA will use the following input files.

File	Contents
RYSRW	Data base for the fast direct evaluation of roots and weights of the Rys polynomials. This file is a part of the program system and should not be manipulated by the user.
ABDATA	Data base for the evaluation of roots and weights of high order Rys polynomial. This file is a part of the program system and should not be manipulated by the user.
ONEINT	One-electron integrals and auxiliary information.
RUNFILE	Auxiliary data generated by seward and gateway.

The files of the SEWARD, SCF, RASSCF, MBPT2, MOTRA, CCSDT, and CASPT2 modules are needed for the numerical procedure.

8.1.5.2 Output files

In addition to the standard output unit ALASKA will generate the following files.

File	Contents
RUNFILE	The runfile is updated with information needed by the SLAPAF relaxation program. ALASKA will write the molecular Cartesian gradients on this file.
ALASKA.INPUT	File with the latest input processed by ALASKA.

8.1.6 Input

Below follows a description of the input to ALASKA. Note that input options are related to the analytic gradient procedure if not otherwise noted!

In addition to the keywords and the comment lines the input may contain blank lines. The input is always preceded by the program name:

  &ALASKA
The first four characters of the keywords are decoded while the rest are ignored. However, for a more transparent input we recommend the user to use the full keywords.

Optional keywords for analytical gradients

Keyword	Meaning
ONEOnly	Compute only the nuclear repulsion and one-electron integrals contribution to the gradient. The default is to compute all contributions to the molecular gradient.
CUTOff	Threshold for ignoring contributions to the molecular gradient follows on the next line. The default is 1.0d-7. The prescreening is based on the 2nd order density matrix and the radial overlap contribution to the integral derivatives.
TEST	With this keyword the program will process only the input. It is a debugging aid to help you check your input.
POLD	The gradient is printed in the old format. Note: by default gradient is not printed any longer.
PNEW	The gradient is printed in the new human-readable format.
VERBose	The output will be a bit more verbose.
SHOW gradient contributions	The gradient contributions will be printed.

Optional keywords for numerical gradients

Keyword	Meaning
NUMErical	Forces the use of numerical gradients even if analytical ones are implemented. The default is to use analytical gradients whenever possible.
ROOT	For use with numerical gradients only! Specifies which root to optimize the geometry for, if there is more than one root to choose from. In a RASSCF optimization, the default is to optimize for the same root as is relaxed. In a MS-CASPT2 calculation, the default is to optimize for root 1.
DELTa	For use with numerical gradients only! The displacement for a given center is chosen as the distance to the nearest neighbor, scaled by a factor. This factor can be set through the DELTa keyword. The default value is 0.005.

The following is an example of an input which will work for almost all practical cases. Note that it is very rarely that you need to run this program explicitly. It is usually controlled by the program Slapaf.

  &Alaska