In this chapter ve shall discuss the calculation of potential energy associated vith molecular conforaations. Raving obtained cartesian atoaic coordinates defining conforaations. and lists of intraao- cular interactions, as described in Chapter 3, ve are ready to calculate a quantity which in the chemical literature is known as the total aolecular potential energy or the conforaational, steric, strain or intraaolecular energy. ~he conformational energy of a aolecule can be expressed as a function , of all internal coordinates and interatoaic distances, or as a function of atoaic positions specified by soae general coordi Dates. ~he function , is supposed to haye local ainiaa corresponding to the stable equilibrium conforaations of a aolecule in vacuo, Deglecting interaolecular interactions. ~he exact fora of Y is. of course, unknown. We assume that it can be suitably approximated by a sua of different types of energy contri hutions: , = Y ., +, +, ., be. nb e ~he teras represent cODtributions to the total aolecular potential energy , due to bond stretching and coapression teras Vb' valence aDgle bending teras 'e' iDterDal rotational or torsional teras V,. DOD-bonded interactions 'nb and electrostatic or Couloab iDter actions 'e. If there are other intraaolecular aechanisas affecting 79 V, sucD as hydrogen bonding, corresponding terms say be added.

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1 Introduction
1.1 What the CFF is
1.2 Background
1.3 Pre-CFF
1.4 The ascent of CFF
1.5 In the wake of CFF
2 The Programming System
2.1 Introduction
2.2 Outline of the programming system
2.3 Other programmes
2.4 Organisation of the system
2.5 Input manual
3 Molecular Topology and Geometry
3.1 Molecular topology
3.2 Lists of interactions
3.3 Molecular geometry
4 The Conformational Energy and its Derivatives
4.1 Introduction
4.2 Intermolecular forces
4.3 Intramolecular forces
4.4 Force field parametrisation
4.5 Energy calculations
4.6 Numerical calculation of derivatives
5 Energy Minimisation
5.1 Statement of the problems
5.2 Minimisation algorithms
5.3 Unified approach to gradient algorithms
5.4 Evaluation of minimisation methods
5.5 The minimisation programme
5.6 Concluding remarks
6 Vibrational Calculations
6.1 The vibrational problem
6.2 Normal coordinates
6.3 Programme VIBRAT
6.4 Practical considerations
7 Optimisation of Energy Parameters
7.1 The basic algorithm
7.2 The partial derivatives
7.3 Implementation of the optimisation
8 Developing A Force Field
8.1 The concept of energy functions
8.2 Examples
9 References.