Run Types

• Runtype control and strategy parameters

The different run types are characterized by how the geometry is manipulated:

SinglePoint

The SCF solution is computed for the input geometry.

GeometryOptimization

The atomic coordinates are varied in an attempt to find a (local) energy minimum. One may let all coordinates free or only a subset, keeping the others frozen at their initial values.

TransitionState

Search for a saddle point. Similar to a GeometryOptimization, but now the Hessian at the stationary point presumably has one negative eigenvalue.

LinearTransit

The geometry is modified step by step from an initial to a final configuration. All of the coordinates or only a subset of them may be involved in the transit. The coordinates to be modified are the LinearTransit parameters. For each of the LinearTransit points (geometries) the computation may be a Single Point SCF calculation or a GeometryOptimization. In the latter case only those coordinates (or a subset of them) can be optimized that are not LinearTransit parameters. The LinearTransit feature can be used for instance to sketch an approximate reaction path in order to obtain a reasonable guess for a transition state, from where a true TransitionState search can be started.

IRC or IntrinsicReactionCoordinate

Tracing a reaction path from a transition state to reactants and/or products. A fair approximation of the transition state must be input. The end-point(s) - reactants / products - are determined automatically.

Frequencies

Computation of force constants and from these the normal vibrational modes and harmonic frequencies. The force constants are calculated by numerical differentiation of the energy gradients at the equilibrium geometry and the slightly deviating geometries (making small displacements of the atoms). No analytical second derivatives have yet been implemented.

Three keys are involved in the specification of the geometry and its manipulation:

ATOMS

sets the atomic (starting) positions.

GEOMETRY

Controls the run type and strategy parameters, such as convergence thresholds and the maximum number of geometry steps to carry out.

ATOMS and GEOMETRY

These two keys together are sufficient for a straightforward Optimization, TransitionState search, IRC run or a Frequencies computation. (Of course, you also need to specify the FRAGMENTS key.

GEOVAR

May be used to impose constraints, for instance when only a subset of all coordinates should be optimized. GEOVAR may also be used in a LinearTransit run to define the LinearTransit parameters and their initial and final values.

Constraints and LinearTransit parameters may also be controlled within the ATOMS block if a MOPAC-style input format is used, see below.

Runtype control and strategy parameters

With the block key GEOMETRY you define the runtype and strategy parameters.

GEOMETRY {RunType { RunTypeData}}

RunType {RunTypeData}

END

RunType

Can be:
- SinglePoint or SP
- GeometryOptimization or GeoOpt or GO
- TransitionState or TS
- IntrinsicReactionCoordinate or IRC
- LinearTransit or LT
- Frequencies or FREQ
If omitted the run type is GeometryOptimization.
If the key GEOMETRY is not used at all the run type is SinglePoint.
The run type specification can be given as argument to the geometry key, or in the data block, but not both. For some run types additional data may be given after the run type specification.

RunTypeData

(Optional) further specifications, depending on the run type. See the sections below.

Omission of the GEOMETRY key altogether effectuates a SinglePoint calculation. A straightforward optimization, with all features that can be set with geometry at their default values, is activated by supplying the key with an empty block:

GEOMETRY

END

More subkeys are available in the GEOMETRY block than just the run type specification. They are used to control strategy parameters such as convergence criteria. All subkeys are optional: default values take effect for those omitted. Some of the subkeys are only meaningful for certain run types. They will be ignored for other run types.

The initial approximation of the Hessian matrix, which may affect the number of optimization steps that are carried out to reach convergence, is not controlled by the key GEOMETRY, but by the key HESSDIAG, and/or by the key GEOVAR. See the section Initial Hessian.