Use tighter convergence criteria.
If the geometry is already converged with default convergence criteria, then just using tighter convergence thresholds should work. Look at the tight option in the gaussian manual.
Sometimes one will just almost always have one small imaginary frequency (like a few wavenumbers) and it can be very hard to eliminate these, ...
Complementing the @Nike answer:
Can we optimize a molecular structure using Moller-Plesset MP4 method
on GAUSSIAN software?
If yes what keyword do we have to add?
You need to add the corresponding keywords like below:
# opt mp4(sdtq)/6-31++g(d,p) geom=connectivity
From Gaussian site:
This keyword requests that a geometry optimization be ...
Gaussian can do all the way up to MP5 for single-point energies, but analytic gradients are only available for MP2, MP3, MP4(DQ) and MP4(SDQ), the latter including only single, double and quadruple substitutions.
MP4(SDTQ) which is like MP4(SDQ) but also with triple substitutions, and MP5, are listed in the manual as being able to do numerical gradients. ...
You can use OpenBabel directly to obtain the Gaussian input from command line.
OpenBabel supports two types of Gaussian input generation:
Gaussian 98/03 Cartesian Input
Gaussian Z-matrix Input
To run OpenBabel:
obabel [-i<input-type>] <infilename> [-o<output-type>] -O<outfilename> [Options]
Take a look at this answer for ...
vkj's answer addressed the third part of the question. I'll answer the first part.
Which quantum chemistry methods can be used in an external electrostatic field? Also which software supports that? I'm interested in anything from semiempirical (eg XTB) to DFT (eg ωB97, B3LYP-D3) to coupled cluster (eg DLPNO-CCSD(T)). The only starting point I have is a few ...