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STEPPER

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Geometry Optimization with STEPPER

The STEPPER module performs a search for critical points on the potential energy surface of the molecule defined by input using the GEOMETRY directive. Since STEPPER is not the primary geometry optimization module in NWChem the compound directive is required; the DRIVER module is the default. Input for this module is specified within the compound directive,

 STEPPER
   ...
 END

The presence of the STEPPER compound directive automatically turns off the default geometry optimization tool DRIVER. Input specified for the STEPPER module must appear in the input file after the GEOMETRY directive, since it must know the number of atoms that are to be used in the geometry optimization. In the current version of NWChem, STEPPER can be used only with geometries that are defined in Cartesian coordinates. STEPPER removes translational and rotational components before determining the step direction (5 components for linear systems and 6 for others) using a standard Eckart algorithm. The default initial guess nuclear Hessian is the identity matrix.

The default in STEPPER is to minimize the energy as a function of the geometry with a maximum of 20 geometry optimization iterations. When this is the desired calculation, no input is required other than the STEPPER compound directive. However, the user also has the option of defining different tasks for the STEPPER module, and can vary the number of iterations and the convergence criteria from the default values. The input for these options is described in the following sections.

MIN and TS -- Minimum or transition state search

The default is for STEPPER to minimize the energy with respect to the geometry of the system. This default behavior may be forced with the directive

 MIN

STEPPER can also be used to find the transition state by following the lowest eigenvector of the nuclear Hessian. This is usually invoked by using the saddle keyword on the TASK directive (Section 5.10), but it may also be selected by specifying the directive

 TS

in the STEPPER input.

TRACK -- Mode selection

STEPPER has the ability to ``track a specific mode during an optimization for a transition state search, the user can also have the module track the eigenvector corresponding to a specific mode. This is done by specifying the directive

 TRACK [nmode <integer nmode default 1>]

The keyword TRACK tells STEPPER to track the eigenvector corresponding to the integer value of <nmode> during a transition state walk. (Note: this input is invalid for a minimization walk since following a specific eigenvector will not necessarily give the desired local minimum.) The step is constructed to go up in energy along the nmode eigenvector and down in all other degrees of freedom.

MAXITER -- Maximum number of steps

In most applications, 20 stepper iterations will be sufficient to obtain the energy minimization. However, the user has the option of specifying the maximum number of iterations allowed, using the input line,

 MAXITER <integer maxiter default 20>

The value specified for the integer <maxiter> defines the maximum number of geometry optimization steps. The geometry optimization will restart automatically.

TRUST -- Trust radius

The size of steps that can be taken in STEPPER is controlled by the trust radius which has a default value of 0.1. Steps are constrained to be no larger than the trust radius. The user has the option of overriding this default using the keyword TRUST, with the following input line,

 TRUST <real radius default 0.1>

The larger the value specified for the variable radius, the larger the steps that can be taken by STEPPER. Experience has shown that for larger systems (i.e., those with 20 or more atoms), a value of 0.5, or greater, usually should be entered for <radius>.

CONVGGM, CONVGG and CONVGE -- Convergence criteria

Three convergence criteria can be specified explicitly for the STEPPER calculations. The keyword CONVGGM allows the user to specify the convergence tolerance for the largest component of the gradient. This is the primary convergence criterion, as per the default settings, although all three criteria are in effect. this default setting is consistent with the other optimizer module DRIVER. The input line for CONVGGM has the following form,

  CONVGGM <real convggm default 8.0d-04>

The keyword CONVGG allows the user to specify the convergence tolerance for the gradient norm for all degrees of freedom. The input line is of the following form,

  CONVGG <real convgg default 1.0d-02>

The entry for the real variable <convgg> should be approximately equal to the square root of the energy convergence tolerance.

The energy convergence tolerance is the convergence criterion for the energy difference in the geometry optimization in STEPPER. It can be specified by input using a line of the following form,

  CONVGE <real convge default 1.0d-04>

Backstepping in STEPPER

If a step taken during the optimization is too large (e.g., the step causes the energy to go up for a minimization or down for a transition state search), the STEPPER optimizer will automatically ``backstep and correct the step based on information prior to the faulty step. If you have an optimization that "backsteps" frequently then the initial trust radius should most likely be decreased.

Initial Nuclear Hessian Options

Stepper uses a modified Fletcher-Powell algorithm to find the transition state or energy minimum on the potential energy hypersurface. There are two files left in the user's permanent directory that are used to provide an initial hessian to the critical point search algorithm. If these files do not exist then the default is to use a unit matrix as the initial hessian. Once Stepper executes it generates a binary dump file by the name of name.stpr41 which will be used on all subsequent stepper runs and modified with the current updated hessian. The default file prefix is the "name" that is used (see START). It also stores the information for the last valid step in case the algorithm must take a "backstep". This file is the working data store for all stepper-based optimizations. This file is never deleted by default and is the first source of an initial hessian. The second source of an inital hessian is an ascii file that contains the lower triangular values of the initial hessian. This is stored in file name.hess, where "name" is again the default file prefix. This is the second source of an initial hessian and is the method used to incorporate an initial hessian from any other source (e.g., another ab initio code, a molecular mechanics code, etc.,). To get a decent starting hessian at a given point you can use the task specification task scf hessian, with a smaller basis set, which will by default generate the name.hess file. Then you may define your basis set of choice and proceed with the optimization you desire.