mepgs

MEPGS

The MEPGS module performs a search for the two critical points on the potential energy surface connected to a saddle point of the molecule defined by input using the GEOMETRY directive (see Section geometry ). The algorithm programmed in MEPGS is a constrained trust region quasi-newton optimization and approximate energy Hessian updates.

Optional input for this module is specified within the compound directive,

      MEPGS 
        OPTTOL <real opttol default 3e-4>
        EPREC <real eprec default 1e-7>
        STRIDE <real stride default 0.1>
        EVIB <real evib default 1e-4>
        MAXMEP <integer maxiter default 250>
        MAXITER <integer maxiter default 20>
        INHESS <integer inhess default 2>
        (BACKWARD || FORWARD) <string default both>
        (MSWG || NOMSWG) <string default NOMSWG>
        (XYZ || NOXYZ) 
      END

Convergence criteria

The user may request a specific value for the tolerance with the keyword OPTTOL which will couple all the convergence criteria in the following way:

             GRMS   1.0*OPTTOL
             GMAX   1.5*OPTTOL
             XRMS   4.0*OPTTOL
             XMAX   6.0*OPTTOL

Available precision

    EPREC <real eprec default 1e-7>

In performing a constrained trust region optimization the precision of the energy is coupled to the convergence criteria (see Section TROPT ). Note that the default EPREC for DFT calculations is 5e-6 instead of 1e-7.

Controlling the step length

        STRIDE <real stride default 0.1>

A dynamic stride (stride) is used to control the step length during the minimum energy path walking when taking the Euler step as starting point.

Moving away from the saddle point

        EVIB <real evib default 1e-4>

The expected decrease in energy (evib) assuming a quadratic approximation around the saddle structure to be obtained.

Maximum number of MEPGS steps

        MAXMEP <integer maxmep default 250>

By default at most 250 minimum energy path steps will be taken, but this may be modified with this directive.

###Maximum number of steps

        MAXITER <integer maxiter default 20>

By default at most 20 constrained geometry optimization steps will be taken, but this may be modified with this directive.

###Initial Hessian

        INHESS <integer inhess default 2>

With this option the MEPGS module will be able to transform Cartesian Hessian from previous frequency calculation.

Selecting the side to traverse

        (BACKWARD || FORWARD) <string default both>

With this option the MEPGS module will select which side of the minimum energy path to explore. By default both sides are explored for a MEPGS run.

Using mass

        (MSWG || NOMSWG) <string default NOMSWG>

With this option the MEPGS will trigger the use of mass when following the minimum energy path. Mass is not used as default, if mass is used then this formally becomes an intrinsic reaction coordinate.

Minimum energy path saved XYZ file

        XYZ [<string xyz default $fileprefix>]
        NOXYZ

The XYZ directive causes the geometry at each calculated structure on the minimum energy path to be output into file in the permanent directory in XYZ format. The optional string will prefix the filename. The NOXYZ directive turns this off.

For example, the input

        mepgs; xyz ; end

will cause a trajectory file filename.xyz to be created in the permanent directory.

###MEPGS usage

        start somename
        geometry; <saddle point body > ; end   
        task theory freq
        freq; reuse somename.hess ; end
        mepgs; <mepgs options> ; end
        task theory mepgs

In the above example, after performing a frequency analysis for the saddle point, the information of the force constant matrix is reused (freq directive) in order to be able to follow the transition state mode.

Example input and output files can be find at https://github.com/nwchemgit/nwchem/blob/master/QA/tests/mep-test/mep-test.nw https://github.com/nwchemgit/nwchem/blob/master/QA/tests/mep-test/mep-test.out