Overflow in LJcorrection?

From: Brian Radak (bradak_at_anl.gov)
Date: Thu Dec 03 2015 - 15:45:38 CST

I've been debugging/testing alchemical corrections to the LJcorrection
when I stumbled upon this weird behavior:

For largish systems (I just threw and asp in a big box, 163737 atoms),
the average A and B coefficients comes back /negative/. This seems to go
away (sometimes?) if I zero out the hydrogen LJ epsilon parameter in the
parameter file. Some strategic print statements seem to suggest that the
"sumOfAs" and "sumOfBs" counters go negative when the next increment is
above ~1e12?

Here is output for vdw type 60 (OT from TIP3P) and the counters for
sumOfAs and sumOfBs when it first goes negative.
The increments should be (54469-1)*54469*A/B

pair : 60/60 : A = 581924 : B = 595.015 | counts : 54469/54469

The second column should be: 2.229940435e+14 and 1.8239593e+12

Does this look like overflow? I can't imagine a double precision value
having trouble here, although the atom counts are 32 bit signed integers
(or whatever the compiler understands "int" to be).


Brian Radak
Theta Early Science Program Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory
9700 South Cass Avenue
Building 240, 1.D.16
Lemont, IL 60439-4871
Tel: 630/252-8643
email: bradak_at_anl.gov

This archive was generated by hypermail 2.1.6 : Thu Dec 31 2015 - 23:22:17 CST