[Known Bug] Warning for numerical orbital users

General discussion of the Cambridge quantum Monte Carlo code CASINO; how to install and setup; how to use it; what it does; applications.

[Known Bug] Warning for numerical orbital users

Postby Ryan_Hunt » Fri Nov 18, 2016 6:06 pm

Dear fellow CASINO users,

I am writing this post to make anybody who might be using numerical orbitals in their QMC calculations aware of a bug we (myself and Neil Drummond) have found. If you have compiled CASINO on a version of gfortran >= 4.9, and are using numerical orbitals in DMC, there is a high chance that you have suffered an issue (in our test case, the DMC energy of Neon with a numerical basis, as found in CASINO/examples/atom/neon_n, was found to lie 4 a. u. lower than the current best estimate in a non-bugged version - which is consistent with earlier values reported in the literature). This is a problem that arises from a known point in the source code and it will be fixed in the near future.

A temporary workaround to this problem, which does not require installation of a new (or in this case old) gfortran version, is to make the following subsitution in the file "CASINO/arch/data/f90/gnu.arch" : the "-Ofast" flag should be replaced by -> "-O3 -ffast-math -fstack-arrays".

I.e. the line
Code: Select all
FFLAGS_opt   = -Ofast -fpeel-loops -fcray-pointer

should be replaced with the line
Code: Select all
FFLAGS_opt   = -O3 -ffast-math -fstack-arrays -fpeel-loops -fcray-pointer

A newly compiled version of CASINO with this change made should not suffer this bug. Hope nobody has been stung by this, and good luck in your work!

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Joined: Thu Mar 10, 2016 10:58 am
Location: Lancaster, UK

Re: [Known Bug] Warning for numerical orbital users

Postby Mike Towler » Mon Nov 21, 2016 12:10 pm

Ryan has now modified the standard gfortran compiler flags in the current beta CASINO distribution so that the problem no longer occurs for that compiler.

The problem lies in the -fno-protect-parens option which is implied by the -Ofast flag; this allows the compiler to disrespect mathematical parantheses under certain circumstances in an attempt to make the code go faster - but that can clearly lead to numerical problems and hence the wrong answer, crashing etc.. Very dangerous!

I fixed a similar problem with the Intel compiler a few years ago (patch 2.13.216); if anyone is aware of related issues with other compilers let me know.

Mike Towler
Posts: 234
Joined: Thu May 30, 2013 11:03 pm
Location: Florence

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