At high cutoff: "ERROR: CALCULATED VDWA FORCE MAY NOT MATCH ENERGY! POSSIBLE BUG!"

From: Victor Kwan (vkwan8_at_uwo.ca)
Date: Sat Jun 30 2018 - 15:35:08 CDT

Hello all,

I am trying to investigate whether long range electrostatic interactions
changes the result of my calculation. Unfortunately MSM and PME is not
an option for my system. When setting cutoff to 400 Angstrom, I've found
the following in the log file:

Info: ABSOLUTE IMPRECISION IN FAST TABLE ENERGY: 1.65436e-23 AT 399.68
Info: RELATIVE IMPRECISION IN FAST TABLE ENERGY: 5.30522e-17 AT 399.68
Info: INCONSISTENCY IN FAST TABLE ENERGY VS FORCE: 0.000290475 AT 0.251946
Info: ABSOLUTE IMPRECISION IN VDWA TABLE ENERGY: 2.53164e-47 AT 399.68
Info: RELATIVE IMPRECISION IN VDWA TABLE ENERGY: 2.63784e-15 AT 397.109
Info: ABSOLUTE IMPRECISION IN VDWA TABLE FORCE: 5.34553e-51 AT 397.109
Info: RELATIVE IMPRECISION IN VDWA TABLE FORCE: 1.74517e-16 AT 397.109
Info: INCONSISTENCY IN VDWA TABLE ENERGY VS FORCE: 1.15649 AT 398.397
ERROR:
ERROR: CALCULATED VDWA FORCE MAY NOT MATCH ENERGY! POSSIBLE BUG!
ERROR:
Info: ABSOLUTE IMPRECISION IN VDWB TABLE ENERGY: 9.86076e-32 AT 399.68
Info: RELATIVE IMPRECISION IN VDWB TABLE ENERGY: 6.31555e-17 AT 399.68
Info: ABSOLUTE IMPRECISION IN VDWB TABLE FORCE: 2.40741e-35 AT 397.109
Info: RELATIVE IMPRECISION IN VDWB TABLE FORCE: 1.933e-16 AT 397.109
Info: INCONSISTENCY IN VDWB TABLE ENERGY VS FORCE: 2.55495 AT 398.397
ERROR:
ERROR: CALCULATED VDWB FORCE MAY NOT MATCH ENERGY! POSSIBLE BUG!
ERROR:

I understand that NAMD uses interpolation table to calculate force and
energy. But what is the implication here? Is the energy output (KINETIC,
TOTAL, TOTAL2 etc.) _potentially_ unreliable? I assume the imprecision
is caused by floating-point error.

Best wishes,

Victor Kwan

This archive was generated by hypermail 2.1.6 : Thu Dec 05 2019 - 23:19:58 CST