# 6.1. CHARMM Improper¶

## 6.1.1. Functional Form¶

The CHARMM improper potential has the functional forms:

$$E={{K}_{d,ijkl}}\left[ 1+\cos \left( N{{\phi }_{ijkl}}-{{\phi }_{0,ijkl}} \right) \right]$$

$$E={{K}_{d,ijkl}}\left[ 1+\cos \left( N{{\phi }_{ijkl}}+{{\phi }_{0,ijkl}} \right) \right]$$

The force-field parameters for this potential and units are given by:

 Equation Symbol Parameter Definition Units $$K_{d,ijkl}$$ Improper coefficient for atoms [i,j,k,l] energy $$N$$ Nonnegative integer coefficient N/A $$\phi_{0,ijkl}$$ Equilibrium improper angle for atoms [i,j,k,l] degrees

## 6.1.2. XML Schema¶

The XML schema for the CHARMM improper potential has the following representation (design mode representation using Liquid XML Studio):

The relationship between the equation symbols and XML schema notations are given by:

 Parameter Definition Equation Symbol Schema Notation Atom type of atom [i] $$i$$ AT-1 Atom type of atom [j] $$j$$ AT-2 Atom type of atom [k] $$k$$ AT-3 Atom type of atom [l] $$l$$ AT-4 Improper coefficient for atoms [i,j,k,l] $$K_{d,ijkl}$$ Kd Nonnegative integer coefficient $$N$$ N Equilibrium improper angle for atoms [i,j,k,l] $$\phi_{0,ijkl}$$ Phi0

The general attributes (describing the entire data set) are given by:

 General Attributes Cardinality Value style Fixed CHARMM formula Fixed Kd*[1+cos(N*Phi-Phi0)], Kd*[1+cos(N*Phi+Phi0)] convention Optional Enumerations specified in schema Kd-units Required Enumerations specified in schema Phi0-units Required Enumerations specified in schema

The specific attributes (attached to each set of parameters) are given by:

 Specific Attributes Cardinality Definition comment Optional Comment attached to parameter set version Optional Version number of parameter set reference Optional Reference attached to parameter set

Note that an XML document will be rejected from being entered into the WebFF database if a required attribute is left unspecified.