Re: LJcorrection option in NAMD

From: Jérôme Hénin (
Date: Tue Jun 21 2016 - 09:48:12 CDT

Brian, thanks for outlining those issues, in particular missing docs. I
can't recommend enough that the docs be updated at the same time as the
features themselves. It's vital for users of development versions, and it's
the best time to do it as a developer as well - not least, it removes the
risk that some things never get documented at all, like the WCA features.

Interactions with the pressure control, both through the kinetic pressure
term for vanishing atoms (an issue you mentioned earlier) or long-range
corrections, are among the subtleties that make it so complicated to
maintain a solid alchemy code. Phrases like "should work more or less ok
now" are pretty worrying for a user to read. They are also a reminder that
we lack test cases for this code.

I know that you need to get some science done, but I hope you can find some
time to clean up the existing code, and build tests to ensure its


On 21 June 2016 at 14:36, Brian Radak <> wrote:

> On 06/20/2016 05:09 PM, Yue Qian wrote:
>> Dear NAMD users,
>> I am currently using NAMD to carry out free energy of binding
>> calculations of a protein-ligand system. I have encountered several
>> problems regrading the long-range LJ corrections. I have googled for
>> certain issues but couldn't find relevant, informative discussions.
>> This is what I found on the manual:
>> LJcorrection: Description: Apply an analytical correction to the reported
>> vdW energy and virial that is equal to the amount lost due to switching and
>> cutoff of the LJ potential. The correction will use the average of vdW
>> parameters for all particles in the system and assume a constant,
>> homogeneous distribution of particles beyond the switching distance. See
>> [60] for details (the equations used in the NAMD implementation are
>> slightly different due to the use of a different switching function).
>> Periodic boundary conditions are required to make use of tail corrections.
>> LJcorrection as implemented is inconsistent with vdwForceSwitching.
>> Here are my questions:
>> 1. To turn on the long-range LJ correction, is "LJcorrection yes" is
>> the only thing I need to do? (From the manual, I noticed that "LJcorrection
>> as implemented is inconsistent with vdwForceSwitching." vdwForceSwitching
>> was kept off.) I have tested this protocol in unbound state calculation
>> with only the solute molecule and TIP3 water molecules with LJcorrection on
>> and off. However, I did not observe significant difference, which was
>> expected to be around 0.5 ~ 0.6 kcal/mol. But with "Ewald off", the
>> calculation result was ~0.8kcal/mol more positive. This patten applies to
>> different cutoffs, from 8 Ang to 12 Ang.
>> Correct, "LJCorrection yes" or "LJCorrection on" will apply the
> correction. As of 2.11 there actually is now a vdwForceSwitching compatible
> correction, I just haven't updated the documentation yet (sorry!). If a
> warning message still states this incompatibility, then it is not present,
> because I also removed that message in the update.
> The new code also improves consistency in conjunction with alchemical
> endpoints, but might not work as expected with the (also mostly
> undocumented) WCA FEP code. Standard FEP and TI should work more or less ok
> now.
> In general, such a correction will not significantly change free energies
> unless the solute rather very large. Even then I might only expect sub
> kcal/mol changes.
> The main changes I expected when I did all of this was to get different
> densities during an NpT equilibration. In general the correction increases
> density by ~5%.
> 2. According to the manual, the LJcorrction option assumes a homogeneous,
>> isotropic environment which only applies to unbound state calculation. To
>> be able to apply to bound state calculation, heterogenous, nonisotropic
>> environment needs to be considered. The cited paper (J. Phys. Chem. B, Vol.
>> 111, No. 45, 2007) covered both scenarios. But GROMACS was used in their
>> work. I was wondering if there's anyway to incorporate such methods in NAMD.
> The main options here are to apply a Lennard-Jones Ewald approach (not
> available in NAMD) or else to use the perturbation-like method by
> re-evaluating the energy with a larger cutoff. The latter is doable but has
> never been automated in NAMD to my knowledge (I suppose we could give this
> a try if there is interest).
>> Any clue is appreciated. Thank you very much.
>> Best,
>> Yue
>> --
>> Yue Qian
> --
> Brian Radak
> Postdoctoral Scholar
> Gordon Center for Integrative Science, W323A
> Department of Biochemistry & Molecular Biology
> University of Chicago
> 929 E. 57th St.
> Chicago, IL 60637-1454
> Tel: 773/834-2812
> email:

This archive was generated by hypermail 2.1.6 : Tue Dec 27 2016 - 23:22:16 CST