[smithm34]

I need to optimize the amino acid Valine in the Zwitterion form. The problem is one of the hydrogens on the nitrogen stretches all the way over to the oxygen.

How do I prevent Gaussian from changes the bond length of the 3 hydrogens on nitrogen?

I think I need to use opt=Z-matrix, but I am unsure how to set it up.

Anyone's help would be greatly appreciated!

[blaisem]

http://www.gaussian.com/g_tech/g_ur/k_geom.htm

It looks like you just type an F (for freeze) after your coordinates on the same line.