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. 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). There is a possibility with the post-ADF program sd, to calculate the second derivatives analytically. Note that this program still has some limitations.

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.

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 or BASIS 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.