# isotope_log_rpfr

Calculate the logarithm of reduced partition function ratio (RPFR) for the specified isotopic substitution using free energy estimators, described in Cheng and Ceriotti (2014) 141(24), 244112.

Thermodynamic estimator is cheap and therefore gets evaluated by default. However, for cases with large relative mass difference between isotopes, scaling_estimator is recommended, since in such cases the thermodynamic estimator has a large variance.

The authors observe (empirically) that starting with the heavy isotopologue gives faster convergence.

This command can appear in the global context.

### Options

atom_list

List equivalent atoms to be substituted by an isotope with mass given in new_mass.

This option is mandatory.

• The type is [int]
• There is no default value.
• The value must be greater than or equal to 1.0
new_mass

Specify the mass of the new isotope in atomic mass units. Atoms in atom_list will be substituted one-by-one, each time outputting the value of the estimator.

This option is mandatory.

• The type is quantity
• There is no default value.
scaling_estimator

Scaling coordinate estimator is more costly to evaluate than the (default) thermodynamic estimator, since if involves an additional evaluation of interatomic potential. However, it is recommended for cases with large relative mass difference between isotopes (e.g. isotopes of Hydrogen), since in such cases the thermodynamic estimator has a large variance.

• The type is bool
• The default is false