# Run Types¶

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. 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
Calculation of normal vibrational modes and harmonic frequencies via numerical differentiation. It is also possible to compute the harmonic frequencies analytically.
CINEB
Calculation of the reaction path and transition state search using Climbing-Image Nudged Elastic Band method. This method is further referrer to as NEB or CI-NEB. Using this method one can find a transition state between two known states, further referred to as initial and final states. The choice which state is initial and which is final is arbitrary. During calculation with this method, a number of replicas, or images, of the system is calculated. These images can be considered as being linked by an elastic band. Each image is optimized in such a way that on each step the forces parallel to the reaction path are removed and spring forces are added that keep distances to this image’s neighbors equal. At the end of the optimization the images are evenly distributed along the reaction path, the image highest in energy being the transition state (if the climbing-image option is on, the default).

For all features that involve changes in geometry, i.e. all run types except the SinglePoint, it is imperative that you use single-atom fragments. Larger molecular fragments can only be applied in SinglePoint calculations.

Four 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.
Constraints
May be used to impose constraints for geometry optimizations, LT, and TS, in the new branch for optimization, which is the default for these optimizations. This key can not be used for IRC, NEB, or for the old branch of optimizations.
Geovar
May be used to impose constraints, for instance when only a subset of all coordinates should be optimized. This key should be used for IRC, NEB, or for the old branch of optimizations, to impose constraints. GeoVar may also be used in a LinearTransit or NEB run to define the LinearTransit or NEB parameters, respectively, and their initial and final values.

Constraints and LinearTransit parameters in the old branch of optimizations may also be controlled within the atoms block if a MOPAC-style input format is used, see below.

## RunType control¶

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
• CINEB

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 following sections.

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 may affect the number of optimization steps that are carried out to reach convergence. See the section Initial Hessian.