Re: Overflow in LJcorrection?

From: Peter Freddolino (
Date: Thu Dec 03 2015 - 15:55:49 CST

Hi Brian,
Out of curiosity, is this dependent on having FEP or related settings on, or no?

> On Dec 3, 2015, at 4:45 PM, Brian Radak <> wrote:
> 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
> 5.0349e+13-->-7.22533e+14
> 5.86584e+10-->-7.3161e+11
> 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).

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