# Release66:Qmmm density

### From NWChem

density [espfit] [static] [dynamical] default dynamical

This directive controls the electrostatic representation of *fixed* QM region
during optimization/dynamics of classical regions. It has no effect
when position of QM atoms are changing.

**dynamical**is an option where QM region is treated the standard way, through the recalculation of the wavefunction calculated and the resulting electron density is used to compute electrostatic interactions with classical atoms. This option is the most expensive one and becomes impractical for large scale optimizations.

**static**option will not recalculate QM wavefunction but will still use full electron density in the computations of electrostatic interactions.

**espfit**option will not recalculate QM wavefunction nor it will use full electron density. Instead, a set of ESP charges for QM region will be calculated and used to compute electrostatic interactions with the MM regions. This option is the most efficient and is strongly recommended for large systems.

Note that both "static" and "espfit" options do require the presence of the movecs file. It is typically available as part as part of calculation process. Otherwise it can be generated by running qmmm energy calculation.

In most calculations default value for **density** would **dynamical**, with the exception of free energy calculations where default setting **espfit** will be used.