I am facing problems getting a reasonable optimized structure from DFT(b3lyp)/6-31g** calculations. The structure I get after optimization has two adjacent rings fused together. I have tried doing all possible remedies like trying different starting geometries, using different convergence criteria, etc. but the end result is always the same except for the case of RHF where the optimized structure is not so bad (the two adjacent rings are not fused). I am unable to understand why would it show such distortion with b3lyp. Can anyone suggest what might be the problem?
Try optimizing with a different basis or method (like MP2), then feed that structure (and probably also wave function) into your preferred theoretical level. If the structure you want won't converge at all, then maybe it isn't really an accurate structure.
Archna Sharma , what quantum mechanics software are you using? Decreasing the convergence criteria is a bad idea. Where did you get the steroid structure, a crystal data bank or did you draw it? If you obtained the structure from a crystal data bank a partial optimization (by freezing other atoms) may be enough. Changing the level of theory or using a mixed basis set may help. Convergence issues are mainly poor starting geometry.
If you post your structure I'd be happy to have a play with it in CASTEP. A CIF file would be ideal, or XYZ with lattice vectors, but I can probably cope with most formats.
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