\(\renewcommand{\AA}{\text{Å}}\)

# bond_style mm3 command

## Syntax

    bond_style mm3

## Examples

    bond_style mm3
    bond_coeff 1 100.0 107.0

## Description

The mm3 bond style uses the potential that is anharmonic in the bond as defined in (Allinger)

\[E = K (r - r_0)^2 \left[ 1 - 2.55(r-r_0) + \frac{7}{12} 2.55^2(r-r_0)^2 \right]\]

where \(r_0\) is the equilibrium value of the bond, and \(K\) is a prefactor. The anharmonic prefactors have units \(\AA^{-n}\): \(-2.55 \AA^{-1}\) and \(\frac{7}{12} 2.55^2 \AA^{-2}\). The code takes care of the necessary unit conversion for these factors internally. Note that the MM3 papers contain an error in Eq (1): \(\frac{7}{12} 2.55\) should be replaced with \(\frac{7}{12} 2.55^2\)

The following coefficients must be defined for each bond type via the bond_coeff command as in the example above, or in the data file or restart files read by the read_data or read_restart commands:

- \(K\) (energy/distance^2)

- \(r_0\) (distance)

## Restrictions

This bond style can only be used if LAMMPS was built with the YAFF package. See the Build package doc page for more info.

## Related commands

bond_coeff

## Default

none

(Allinger) Allinger, Yuh, Lii, JACS, 111(23), 8551-8566 (1989),