41.8 Thermodynamical properties (THERMO)

It is also possible to calculate the thermodynamical properties of the molecule. Since MOLPRO can only handle Abelian point groups it is necessary to give the point group of the molecule in the input file:

THERMO,SYM=pointgroup

pointgroup has to be the Schoenflies Symbol (e.g. C3v for ammonia; linear molecules have to be C*v or D*h respectively). If no point group card is given, rotational degeneracy will be set to 1, eventually causing deviations in the rotational entropy. If no other input card is given the zero-point vibrational energy and the enthalpy $H(t)-H(0)$ [kJ/mol], heat capacity $C_v$ [J/mol K] and entropy $S$ [J/mol K] are calculated for standard temperature and pressure ($T=298.150$ [K], $p=1$ [atm]).

Subcommands of THERMO are

[PRINT,THERMO] additional information (such as atomic masses, partition functions and thermodynamical function in calories) is printed to the output. [SCALE,factor] in calculating the thermodynamical properties use vibrational frequencies scaled with factor, in order to take account of systematic errors of the wavefunction (e.g. using SCF wavefunctions factor=0.89 is reasonable).

TEMP,tmin,tmax,tstep

calculate the thermodynamical properties at different temperatures, starting with tmin [K] up to tmax [K] in steps of tstep [K].

PRESSURE,p

calculate the thermodynamical properties at a given pressure of p [atm].

The FREQUENCIES program sets the variable zpe containing the zero-point-energy of the harmonic vibrations in atomic units. If the THERMO option is used, the variables htotal and gtotal, containing the enthalpy and the free enthalpy of the system in atomic units, are also set.