Structure and Reactivity
************************

      **See also:**
      ADF-GUI tutorials: geometry optimization, vibrational frequencies, spin coupling in Fe4S4
      Examples: geometry optimization, reactivity


Single point calculations
=========================

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

* Single point calculations section of the AMS manual.


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 eigenvalues. With AMS one can do an optimization
using delocalized or Cartesian coordinates. Constraints (also block
constraints) and restraints are possible. An initial Hessian can be
provided.

* Geometry Optimization section of the AMS manual.


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 eigenvalue.

* Transition state search section of the AMS manual.

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.

* Linear Transit, PES scan section of the AMS manual.


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.

* Nudged Elastic Band (NEB) section of the AMS manual.


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.

* Intrinsic Reaction Coordinate (IRC) section of the AMS manual.


Excited state optimizations
===========================

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

* Excited state gradient section of the ADF manual,

* Excited state optimizations section of the AMS manual.


Molecular dynamics
==================

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

* Molecular dynamics section of the AMS manual.


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 is the sign.

* Nuclear gradients section of the AMS manual.


Hessian
=======

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

* Hessian section of the AMS manual.

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.

* PES point character section of the AMS manual.


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
