[extremesonic]

Hello

I want to perform the relaxed scan for the valence angle energy of Si–O–Ca, Si–O–Mg, Al–O–Ca, and Al–O–Mg. I will use DFT (ORCA), probably B3LYP and triple zeta basis set (Def2-TZVP or 6-311++G(2df,2pd)) I'm making reference data to parametrize an empirical force field.

But the problem is metal ions, Mg²⁺ and Ca²⁺, because their coordination chemistry is very different from Si or Al. More precisely, I'm not sure how can I 'terminate' the Mg or Ca atoms.

I was thinking of making simple molecules, such as (OH)₃–Si–O–Mg–OH or (OH)₂–Al–O–Mg–OH, just giving one OH to Mg or Ca. In this case, Mg or Ca have two oxygen neighbors. I can perform geometry optimization for those molecules, but I'm not sure if such molecules are proper one to perform valence angle scan or not. Are those molecules good enough to check angle energy of Si-O-Mg or Al-O-Ca? Or should I need to use other molecular geometry?

I'm not considering aqueous environment (because this is for condensed phase material), so I'm not planning to use water molecules at current stage. But maybe I need to use water, if there are no other way.

Welcome any idea. Thank you