My molecule includes C,H,O,N,Si
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2$\begingroup$ Generally, for anything that large the first course of action would be to try a generic force field calculation. It also greatly depends on the kind of structures. You'd have to give us more information to help you. $\endgroup$– FractonCommented Feb 3 at 5:24
1 Answer
First, "best" is ill-defined. The best method would be full configuration interaction at the complete basis set limit including fully relativistic and quantum field theoretical corrections; however, such calculations are prohibitively expensive even for small molecule.
For systems of your size, geometry optimization doesn't even really make sense. A molecule of 8000 atoms likely has many floppy modes, meaning that it has a huge number of conformations accessible at room temperature. Reliable simulation of such molecules requires statistical sampling through molecular dynamics or Monte Carlo simulations.