AMBER Archive (2002)

Subject: Calculating atomic charges for AMBER with Gaussian98W

From: Thomas Steinbrecher (thomas.steinbrecher_at_physchem.uni-freiburg.de)
Date: Sat Nov 23 2002 - 07:52:47 CST

Dear Amber users,

I have a question concerning the calculation of atomic
charges for small organic molecules with RESP charge
fitting.

I use Antechamber with the -fo gcrt option to create a
Gaussian Job file. I plan to run this with Gaussian98W,
to use the output with Antechamber again to create a .prep
file, as suggested in the manual on p. 70-71 and p. 259.

My problem is that the Gaussian runs take very long
(Running on a 433MHz Intel computer with 128 MB RAM and
plenty of disk space. My molecule contains ca. 20 heavy and
30 hydrogen atoms). If I check the run after 24 hours, I
find the following in my .out file:

"...
 Item Value Threshold Converged?
 Maximum Force 0.078301 0.000450 NO
 RMS Force 0.006788 0.000300 NO
 Maximum Displacement 0.852774 0.001800 NO
 RMS Displacement 0.155835 0.001200 NO
..."

I assume this means, that the geometry optimization is far
from being complete because the values are so much larger
than the Thresholds.

My question is, is this behaviour to be expected?

How long do Gaussian Runs from the antechamber created job
files approximately take for "typical" organic ligands (no
metal atoms, ca 20 C, O and N atoms)?

Is there any way to speed up the calculations? I thought
about removing the SCF=tight command from my G98 input
file, but I'm not sure if the atomic charges calculated
that way are still valid for use in AMBER.

Sorry for the length of my post.

Kind Regards,

Thomas