Gaussian already includes NMP as a predefined solvent in its solvation models like PCM, IEFPCM, and SMD.

So, the most robust and accurate way is to specify it by name, like this: SCRF=(SMD, Solvent=NMP). Similarly for others

Umer Sherefedin Yasin

Thank you for answering my question.

I tried using scrf=(smd,solvent=nmp) as a solvation model, but my calculation crushed due to an error in the line.

Gaussian version is G16 A.03 AVX2

Everything turned out to be a bit more complicated. But it is quite possible. Professor Zhe Wang has an interesting note on this topic:

https://wongzit.github.io/calculation-of-solvation-free-energy/

Umer Sherefedin Yasin

I think, either NMP really isn't in the Gaussian library, or I'm entering the keyword incorrectly.