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>}
ensemble pimd <num_beads> <T_1> <T_2> <T_coup> {<pressure_control_parameters>} eco <omega_max>
In all forms, 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 or pimd_scr (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.
Appending eco <omega_max> selects the economised path-integral internal-mode frequencies [Zeng2026].
Here omega_max is the highest physical vibrational wavenumber to be reproduced, in units of cm-1.
The dimensionless fitting range is recalculated from the instantaneous target temperature as \(x_{\max}=hc\omega_{\max}/(k_{\rm B}T)\), so Eco frequencies also follow a temperature ramp from T_1 to T_2.
If the eco option is omitted, GPUMD uses the original Trotter internal-mode frequencies.
The Eco option is available for pimd and pimd_scr only; rpmd and trpmd use the original Trotter frequencies.
pimd_scr
If the first parameter is pimd_scr, it means that the current run will use path-integral molecular dynamics (PIMD) with stochastic cell rescaling (npt_scr) for pressure control.
It can be used in the following ways:
ensemble pimd_scr <num_beads> <T_1> <T_2> <T_coup> {<pressure_control_parameters>}
ensemble pimd_scr <num_beads> <T_1> <T_2> <T_coup> {<pressure_control_parameters>} eco <omega_max>
The pressure-control parameters have the same meaning and support the same isotropic, orthogonal, and triclinic forms as in the pressure-controlled pimd case.
The difference is that pimd uses the Berendsen barostat, whereas pimd_scr uses stochastic cell rescaling.
The eco option has the same meaning as described above for pimd.
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.