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.