623. Comparing frequency calculations in G09 and NWChem -- the importance of grid density

The vibrational entropy term will differ between calculations done in gaussian and nwchem unless you use "grid xfine" in nwchem. That the grid density is important is nothing new, but the magnitude of the effect on the entropy surprised me.

The difference can be large -- 30 cal/molK for [PPh4]⁺ at pbe0/cc-pvdz -- which becomes quite significant when multiplied by T (e.g. 298.15 K).

Note that the rotational entropy term also may differ, but that this would be due to different uses of symmetry in the calculations: http://molecularmodelingbasics.blogspot.com.au/2012/12/conformational-and-rotational-entropy.html

If you turn off symmetry (noautosym) in nwchem the rotational entropy will not be corrected. I've noticed that Gaussian, on the other hand, will sneakily apply correction if it finds an acceptable symmetry even if you request nosymm, so make sure that you scan through the output carefully.

Either way, vibrational entropy is not symmetry dependent. Instead you will have to worry about the grid fine-ness when comparing outputs.

If your molecule is very small, such as benzene or tetramethylphosphonium, it seems that you don't have to worry about this. However, even fairly small molecules such as [PPh4]⁺ will be affected.

Conv. Dens. = Convergence Density

Code	Symm	Grid	Conv. Dens.	DFT Energy	ZPE	HCorr	S(tot)	S(trans)	S(rot)	S(vib)
G09	N	F	1E-8	-1266.58424152	0.370516	0.389751	147.076	43.358	35.009	68.708
G09	N	U	1E-8	-1266.58430374	0.370464	0.389691	146.829	43.358	35.009	68.461
NW	N	X	1E-8	-1266.58455223	0.370348	0.389549	146.697	43.339	34.994	68.365
NW	N	X	1E-5	-1266.58455222	0.370348	0.389549	146.704	43.339	34.994	68.371
NW	Y	F	1E-5	-1266.58453684	0.370034	0.385683	118.023	43.339	33.617	41.067
NW	Y	F	1E-8	-1266.58453684	0.370034	0.385683	118.023	43.339	33.617	41.067
NW	N	F	1E-5	-1266.58454928	0.370034	0.385683	119.394	43.339	34.994	41.062
NW	N	F	1E-8	-1266.58454929	0.370034	0.385683	119.394	43.339	34.994	41.061
NW	Y	X	1E-5	-1266.58455274	0.370348	0.389549	145.331	43.339	33.617	68.376
NW	Y	X	1E-8	-1266.58455275	0.370348	0.389549	145.337	43.339	33.617	68.382

F=fine. X=Extrafine. U=Ultrafine.
S(rot) values in blue are symmetry corrected. That's completely normal.

With "grid fine" NWChem gives a very different result to G09.

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You can see the difference in the predicted IR spectra as well.

Fine (NWChem) (blue rings) vs G09 (red circles):

"grid xfine" (NWChem) (blue rings) vs G09 (red circles):