# Calculate *S*_{msRRHO}

How to calculating an *averaged* *S*_{msRRHO} for CRESTâ€™s entropy mode.

## Calculating an averaged *S*_{msRRHO}

In *Chem. Sci.*, **2021**, *12*, 6551-6568 the conformational entropy is calculated as

\(S_\text{conf} = S'_\text{conf} + \bar{S}_\text{msRRHO}\)

where \(S'_\text{conf}\) is the conformational term proportional to the Gibbs-Shannon entropy, and the *averaged* *S*_{msRRHO} is obtained as

\(\bar{S}_\text{msRRHO} = \sum_i^\text{ensemble} p_i S_{\text{msRRHO},i} - S_{\text{msRRHO},0}\)

The *averaged* *S*_{msRRHO} is an ensemble average over all *S*_{msRRHO} contributions for each conformer. Note that the *S*_{msRRHO} entropy *for the input structure*, *S*_{msRRHO,0} is substracted here in order to make the entire term *S*_{conf} an additive correction to the DFT-based molecular entropy.

The *averaged* *S*_{msRRHO} can be calculated in a standalone application of CREST, similar to the `--mdopt`

and `--screen`

commands. The respective calculation can be invoked by the `--rrhoav`

command.

`crest --rrhoav input-ensemble.xyz`

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