Calculation For Non-standard Isotopologues
By default, CFour calculations are carried out for the main isotopologue (for atomic masses and the corresponding standard choice see List of used atomic masses). While for single-point energy calculations, geometry optimizations, and NMR chemical shift calculations the results are independent of the chosen isotopologues, this is not the case for molecular properties such as harmonic frequencies, nuclear spin-rotation constants, or the diagonal Born-Oppenheimer corrections which all depend on the chosen masses.
It is possible to perform with CFour calculations using a non-standard choice for the masses. The following options exist:
a) to run the whole calculations with a non-standard choice of masses, either a %isotope or a %masses section can be supplied in the ZMAT after the keywords.
For example, in the case of water (H2O), the standard choice corresponds to 1H (mass = 1.007825035 amu) and 16O (m = 15.99491463 amu). A calculation for D2O (atomic ordering in the ZMAT O, H, and H) can be performed by adding either
in the ZMAT file after the CFOUR keyword list.
b) in some cases (harmonic frequency calculations and computation of full cubic force fields), it is possible to just repeat the final analysis using a different set of masses. In this case, the required information is either supplied via the file ISOMASS or ISOTOPES (for details, see calculation of harmonic frequencies).