FIX - Changing the sign of the argument of the exponential in LOWER_WALLS

[Iximiel]

Description

This should solve #1091, see the issue for the problem

I want to benchmark the (-x) version against the (abs(x)) version, but maybe that is a thing for another PR.

Shall we add a new test to check the sign in this and maybe other COLVARs?

Target release

I would like my code to appear in release XXXXX

Type of contribution

• [x] changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• [ ] changes to a module not authored by you
• [ ] new module contribution or edit of a module authored by you

Copyright

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• [ ] the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• [ ] I added a new regtest or modified an existing regtest to validate my changes.
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[GiovanniBussi]

I think the abs(x) vs -x has no impact on performance. I would use the most robust one.

Something that instead might be relevant (if you have many of these biases) is that they construct a lot of actions. @gtribello is it necessary? Are you aware that you can define intermediate values in lepton?

A test for it is here though it is not very instructive. But you can do something like:

FUNC=7*a^n;a=(x-x0);x0=3;n=2

The only counterintuitive thing is that values are parsed from right to left, so that you first use quantities then define them.

The example above should do 7*(x-3)^2

The advantage is that:

• there is a single lepton call (performance gain should be measurable if you have many of these)
• lepton simplifies the expression (should be irrelevant in this case)