# Are there any open source software for calculating the rotational and translational entropy of molecules?

What is a good open source software for calculating the rotational and translational entropy of molecules? The National Institute of Standards and Technology does not provide standard molar entropy for all molecules.

## ASE: Thermochemistry module

The thermochemistry module of ASE is able to calculate these for you.

• Just to remember that ASE, by it self, doesn't do any calculation. It needs external calculators. The included calculator EMT is a simple calculator recommended only for quick demonstrations and tests.
– Camps
May 18 '20 at 14:13
• True, but in this case the module actually does a bit of calculating itself. It takes the potential energy and the vibrational energies to calculate the thermodynamic quantities. Of course you still need a code to calculate the former. But ASE interfaces to multiple open source options there. May 19 '20 at 15:42
• Not quite sure I understand what you want to show. It's very unconvenient since the link is not clickable anymore, but if it helps you sure... Jul 1 '20 at 8:06
• @patrickmelix I have converted it back. For some reason, the system still didn't like it. The site has a tour page that uses an example question to explain the site to new user. For whatever reason, its very strict about what sort of formatting these posts can have. I finally found another question that didn't require ugly hacks to get it to fit.
– Tyberius
Jul 3 '20 at 18:37

Any quantum chemical program should calculate these for you as part of a vibrational calculation. Generally, the vibrational, rotational, and translational entropies will be given.

The critical task is to start with an optimized geometry with a good method - and ideally some level of conformer search (i.e., to make sure you have a low-energy conformer).

The rotational entropy derives from the moments of inertia, through the rotational partition function as well as any symmetry of the molecule. So you'll want to make sure you have a fairly accurate geometry, optimized using that particular method, to get accurate moments of inertia.

Here's an example using the xtb program - I'm analyzing these for a paper:

       temp. (K)  partition function   enthalpy   heat capacity  entropy
cal/mol     cal/K/mol   cal/K/mol   J/K/mol
298.15  VIB  0.350E+07            10585.483     74.558     62.428
ROT  0.115E+08              888.752      2.981     35.294
INT  0.404E+14            11474.235     77.539     97.722
TR   0.626E+28             1481.254      4.968     43.410


Here you want the VIB, ROT, and TR entropies for the vibrational, rotational, and translational entropy.

Any molecular quantum program that I've used will spit these out after performing a vibrational calculation. The format of the output varies, though.

• I'm checking - in principal, the cclib package helps parsing entropies, although I haven't used it. May 19 '20 at 17:03
• Better make sure your geometry is a minimum for the method; otherwise the vibrational calculation makes very little sense.. May 24 '20 at 15:56
• @SusiLehtola - 100% agreed - I think it's already above in bold: "critical task is to start with an optimized geometry with a good method". I didn't see this mentioned in the other answers, thus I wrote one. May 25 '20 at 14:40

## Psi4 and NWChem

1. For open source software to perform this task, I can think of Psi4. It's probably the easiest to download/install and get going. It has the usual computational chemistry methods along with ab-initio/SAPT methods.

Some details can be found here: psi4: http://www.psicode.org/ psi4 thermochemistry: http://www.psicode.org/psi4manual/master/thermo.html#examples

1. And of course, there is the beautiful open source NWChem - which is actively developed at PNNL! It can do all sorts of calculations like Gaussian. See for more details: https://nwchemgit.github.io/

## GoodVibes

GoodVibes is able to calculate what you want from Gaussian logfiles. From its documentation (using scaled vibrational frequencies):

python -m goodvibes examples/methylaniline.out -v 0.95

Structure                    E        ZPE             H        T.S     T.qh-S          G(T)       qh-G(T)
*********************************************************************************************************
o  methylaniline      -326.664901   0.135012   -326.521265   0.040238   0.040091   -326.561503   -326.561356
*********************************************************************************************************

• It also seems to supports ORCA and NWChem output files. It's moreover highly customizable and supports approximations beyond RRHO. Mar 13 at 19:16