Miscellaneous Phase Options

Changing the Default Units

By default, gibbs2 understands all the input quantities are in atomic units: Hartree for the energies and frequencies, bohr^3 for the volume, etc. This behavior can be changed with the UNITS option to PHASE:

PHASE ... [UNITS {VOLUME {BOHR|BOHR3|BOHR^3|ANG|ANG3|ANG^3}}
                 {ENERGY {HY|HARTREE|HA|EV|EVOLT|ELECTRONVOLT|RY|RYDBERG}}
                 {PRESSURE {AU|A.U.|GPA}} 
                 { {FREQ|FREQUENCY} {HARTREE|HY|HA|CM-1|CM^-1|CM_1|THZ}}
         {EDOS {HY|HARTREE|HA|EV|EVOLT|ELECTRONVOLT|RY|RYDBERG}}]

The UNITS option to PHASE selects the units for the input volume (VOLUME), energy (ENERGY), pressure (PRESSURE), frequency (FREQ or FREQUENCY) and Fermi energy (EDOS). For instance, if your electronic structure program uses angstrom^3 units for the volume and eV for the energy, then it is convenient to use:

PHASE ... UNITS VOLUME ANG3 ENERGY EV

All data is converted to atomic units internally, and the output is always in atomic units, except where explicitly noted.

Input of pressure-volume data

The PVDATA option to PHASE can be used to indicate that the input data is pressure-volume instead of energy-volume:

PHASE ... PVDATA

The equation of state, in its p(V) form, is fit to the data and then an E(V) curve is generated by integration.

Because p(V) data do not contain points in the negative pressure region, the volume grid needs to be extended somewhat by adding points with \(V > V_0\), where \(V_0\) is the zero-pressure volume. The global keyword NEWPTS is used to control the number of points added in the \(V > V_0\) zone:

SET NEWPTS newpts.i

where newpts.i is the number of points added (default = 20). Likewise, the extent of the expansion in the negative pressure range can be controlled using:

SET FACEXPAND fac.r

The original range [\(V_1\),\(V_0\)] is expanded to [\(V_1\),\(V_0'\)], where:

\(V_0' = V_1 + (V_1 - V_0) \times\) fac.r

By default, fac.r is 0.40.

Energy Shift

PHASE ... [ESHIFT eshift.r]

The ESHIFT option to phase displaces the static energy by eshift.r. The units are the same as in input. By default, there is no shift in the energy.