Dear Harouat Amina ,

The answer to your question is yes. In general, whatever software package (whether it is Gaussian or Turbomole, or VASP) you are using to perform computational calculations on a more extensive system consumes time! Of course, it depends on the computational facility too.

However, you can reduce the time by compromising the accuracy of your results! For example, rather than using a high level of theory, you can use a lower level of theory (caution: sometimes it may lead to utterly wrong or opposite results!!). Please check for the benchmark studies for your system and choose a proper level of theory that balances the accuracy and time.

You can also make small models of your system and qualitatively discuss things. For example, if you have to work on a cluster, you can reduce the cluster size, see whether it matches experimental observations, and proceed further.

Another way to proceed is to use QM/MM calculations, which means treating the specific portion of your system quantum mechanically, and the remaining portion classically. Again, the selection of your two parts (QM part and MM part) should decide with caution. Please refer to a few papers that describe QM/MM methods.

I hope the answer helps! If you carefully think, you can come with several other methods/approximation/techniques without affecting the desired level of accuracy.

Vipin Raj K

Thank you very much for this explanation which really helped me.

Yes, it will depends on the size of the compound.

Hello

Yes it does. Actually, it depends on to your chemical structure and your computer's power

I myself waited for 18 days to get a result. It was really boring.

Dear Harouat Amina , yes it is, just like the answer Vipin Raj K said. Actually, the calculation time is proportional to the number of atoms and electrons. The more atoms or electrons of your system, the longer time would be consumed. (Absolutely, this phenomenon is based on the same computational level and facility.)