# Failing to optimize geometry of tricalcium bis(phospate) with GAMESS [closed]

I'm trying to learn how to use GAMESS. I used Avogadro to download the structure of tricalcium bis(phosphate), then I accidentally ran a runtyp=energy on the structure and it succeeded, so I assumed the resulting structure is better than what avogadro got and used that one instead.

I don't even know if geometry optimization is the first thing I should do. Maybe I should use another runtyp first and then optimize the result?

Maybe the fact that it's a salt is what's getting in the way? Maybe I have to specify the charges of the cations and anions somewhere.

Maybe I have to preoptimize it in Avogadro, because I don't think the straight line of 3 Ca followed by 2 $PO_4$ perfectly aligned remotely resembles what $Ca_3(PO_4)_2$ looks like. (save the pastebin as .log and open it in avogadro so you'll see how it is structured)

Here's a pastebin of the result. The input is printed in the begining, I took off the extended credits section on the begining.

http://pastebin.com/wivjv9WY

## closed as unclear what you're asking by Jannis Andreska, Jan, Wildcat, porphyrin, ringoNov 5 '16 at 21:03

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• Why would you want to optimize a non-existent molecule in the first place? – Ivan Neretin Jul 29 '16 at 7:37
• en.wikipedia.org/wiki/Tricalcium_phosphate – FinnTheHuman Jul 29 '16 at 7:39
• You seem to think that all existing compounds are made of molecules. This is very much not so. The compound $\ce{Ca3(PO4)2}$ does exist, of course, but there is no such molecule. – Ivan Neretin Jul 29 '16 at 7:44
• Can you clarify what do you mean by that? Geometry optimization is only supposed to be done on molecules, is that it? Like I said, I'm learning. I'm not a chemist, My knowledge of chemistry is bellow amateur level. – FinnTheHuman Jul 29 '16 at 7:58
• You really need a more thorough understanding of chemistry first, then you need some decent understanding in quantum mechanics, because you need to combine that to run meaningful computations. Try optimising simple organic molecules first. Use something like water, ammonia, hydrogen chloride, methane, ethane, etc. These have already quite a lot of pitfalls to start with. Obviously you can optimise the geometry of $\ce{Ca3(PO4)2}$, but how meaningful is that as a molecule in the gas phase? – Martin - マーチン Jul 29 '16 at 8:45

Thus, for instance, you have to understand that usually by default all the calculations in quantum chemical programs are done for an isolated molecule in gas phase. Doing geometry optimization for ionic compound in such settings is, well, let's say, it is not so meaningful. Very likely, you want to do something totally different, say, establish the crystal structure of solid $\ce{Ca3(PO4)2}$, but that is a totally different task. In fact, you do even need a special program (like CRYSTAL) to achieve that.