*From*: Igors Mihailovs <igorsm]=[cfi.lu.lv>*Subject*: CCL:G: [CCL]Re: CCL:G: Gaussian 09 and 16 compared for mundane calculations*Date*: Fri, 4 Mar 2022 04:39:28 +0200

Hello Dr. DeYoung,

Personally I have an overall impression that Gaussian 16 is a bit faster either because of algorithm improvements or maybe because it utilizes AVX2 CPU instructions available on our newer nodes. Although I have now checked and for small molecules Gaussian 09 at HF level somehow is consistently a bit faster for the second hyperpolarizability.

In regards to difference in results. The main difference in defaults is the integration grid for DFT which is now UltraFine and not Fine like in Gaussian 09, as well as tighter two-electron integral accuracy. This can indeed cause some serious difference in results (UltraFine is much better). But if we compare HF with various basis sets, for small molecules second hyperpolarizability results were not different more than by 0.002 % for me.

On 04.03.22 00:30, Andrew DeYoung
andrewdaviddeyoung##gmail.com wrote:

Hi,

My institution has both Gaussian 09 and Gaussian 16, and I'm trying to decide which to use for my new project. (My background is in applications of MD simulations, and I'm quite new to quantum chemistry.) One reason to use 16 is that, well, it's the latest version. One reason to use 09 is for consistency; it's been used in our group a lot in the past.

My colleague said he observed some different results from 09 and 16, though he didn't remember how large the differences were.

I will likely do some of my own comparisons, but I am curious, does anyone have experience with this?

The calculations I will be doing for my project are pretty mundane: using RHF to optimize geometry, and using MP2 to calculate the energy and extract charges with CHelpG. The basis sets I will use are fairly pedestrian -- 6-31G(d), cc-pVTZ, etc -- nothing particularly exotic.

From looking at this page ( https://gaussian.com/gdiffs/ ), it looks like Gaussian 16 mainly added new methods and basis sets. I can imagine that algorithm improvements/tweaks (faster, more effective parallelization, etc) could give rise to some numerical differences, but I would not expect these differences to be terribly significant for most properties. Is this your experience?

Thanks for your time,Andrew

Andrew DeYoung, PhDDepartment of ChemistryCarnegie Mellon University