# Structure and Reactivity¶

**See also:**Examples: geometry optimization, reactivity

## Single point calculations¶

A single point calculation calculates one point on the potential energy surface.

## Geometry optimization¶

A geometry optimization tries to find a (local) minimum on the potential energy surface, where the gradients are zero and the Hessian only has positive eigen values. With AMS one can do an optimization using delocalized or Cartesian coordinates. Constraints (also block constraints) and restraints are possible. An initial Hessain can be provided.

## Transition state search¶

A transition state corresponds to a saddle point on the potential energy surface, where the gradients are zero and the Hessian has one negative eigen value.

A reaction coordinate for transition state search can be specified.

## Linear Transit, PES scan¶

The PES scan task in AMS allows users to scan the potential energy surface of a system along one or multiple degrees of freedom, while relaxing all other degrees of freedom. If only one coordinate is scanned, this kind of calculation is usually just called a linear transit.

## Nudged Elastic Band (NEB)¶

The Nudged Elastic Band (NEB) method can be used to find a reaction path and the transition state between a reactant and a product state.

## Intrinsic Reaction Coordinate (IRC)¶

The path of a chemical reaction can be traced from the transition state to the products and/or reactants using the IRC method.

## Excited state optimizations¶

ADF can calculate the nuclear gradient for a particular electronically excited state, which makes it possible to do excited state optimizations.

## Molecular dynamics¶

Molecular dynamics can be used to simulate the evolution of a system in time.

# Gradients, Hessian, Thermodynamics¶

## Gradients¶

A nuclear gradient is the first derivative of the energy with respect to the nuclear coordinates. The nuclear gradients are not forces, the difference being the sign.

## Hessian¶

A Hessian is the second derivative of the energy with respect to the nuclear coordinates.

One can also calculate a partial Hessian. The Hessian calculation is related to the calculation of IR frequencies, see the IR frequencies and normal modes section of the AMS manual.

## PES point character¶

The AMS driver can quickly, and without calculating the full Hessian, characterize a PES point as a local minimum, a transition state, a higher order saddle point, or a non-stationary point.

## Thermodynamics, gas phase Gibbs free energy¶

At the end of a completed IR Frequencies (normal modes) calculation, a survey is given of thermodynamic properties: entropy, internal energy, constant volume heat capacity, enthalpy and Gibbs free energy, see:

IR frequencies section of the AMS manual

Thermodynamics

Gibbs free energy change for a gas phase reaction