CCL: Large Computation Time Leap in Mopac and A Question About pm6



 Sent to CCL by: "Sina  T  reli" [sinatureli!^!gmail.com]
 Hello, have two questions mainly regarding mopac.
 1- Can mndo parameters in a molecular modelling program be replaced with mndo/d
 and pm6 parameters. Or in another words are the functions used in these three
 methods the same and only the parameters different?
 2- I am using mopac2007 to do some geometry optimizations with mndod and pm6. My
 first subject was a molecule of size 141 which was handled quite nicely and fast
 and ended about in 30 minutes. However when I added a ligand that was about of
 20 atoms in size, the calculation time leaped to like 2 days. The cycles werent
 even initiated before half a day passed... I used .mopac as the first output
 file and when I wanted to optimized the ligated one I used .xyz format with some
 atoms fixed to their coordinates. Is there something I am doing wrong or is this
 leap expected? And also I read that mopac 2007 isnt very practical for molecules
 of size above 600. Say if I use it for a molecule of size 400 atoms would it
 take too long on a 3.8 ghz 1gb ram machine? (I might plan to continue the work
 in a advanced computational center then...)
 Thanks