ensemble (PIMD)

The ensemble keyword is used to set up an integration method (an integrator). The integrators described on this page enable one to carry out path integral molecular dynamics (PIMD) simulations and thereby to incorporate quantum dynamical effects.

Syntax

pimd

If the first parameter is pimd, it means that the current run will use path-integral molecular dynamics (PIMD).

It can be used in the following ways:

ensemble pimd <num_beads> <T_1> <T_2> <T_coup>
ensemble pimd <num_beads> <T_1> <T_2> <T_coup> {<pressure_control_parameters>}

In both cases, num_beads is the number of beads in the ring polymer, which should be a positive even integer no larger than 128. The first case is similar to the NVT ensemble with nvt_lan as the Langevin thermostat is used for both the internal and the centroid modes [Ceriotti2010]. The second case is similar to the NPT ensemble with npt_ber, where a Berendsen barostat is added compared to the first case. Note that pimd (that is, not rpmd or trpmd described below) must be the first run that requires to set num_beads and one cannot change num_beads from run to run.

rpmd

If the first parameter is rpmd, it means that the current run will use ring-polymer molecular dynamics (RPMD) [Craig2004].

It can be used as follows:

ensemble rpmd <num_beads>

This can be understood as the NVE version of PIMD, where no thermostat is applied.

trpmd

If the first parameter is trpmd, it means that the current run will use thermostatted ring-polymer molecular dynamics (TRPMD) [Rossi2014].

It can be used as follows:

ensemble trpmd <num_beads>

This is similar to RPMD, but the Langevin thermosat is applied to the internal modes.