Re: [Apbs-users] Example script of grid artifact subtraction
Biomolecular electrostatics software
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From: Nathan B. <ba...@bi...> - 2009-12-11 17:22:31
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Hello -- Actually, all of the input files provided in the APBS examples/ directory demonstrate this in a variety of ways. Some of the examples remove self-energies by calculating solvation energies while other examples remove self-energies by using exactly the same grid positions for all atoms in each part of the calculation. Hope this helps, Nathan On Fri, Dec 11, 2009 at 9:23 AM, Gatti, Domenico <dg...@me...>wrote: > Hi All, > Could we post an EXAMPLE SCRIPT of how the corrections suggested by > Gernot and Nathan for Dziedzic's simple calculation of two point charges > should be implemented? Does this mean that the grid artifact subtraction > should be carried out always in all the APBS calculations? Perhaps, this > occurs already by default, and I did not realize it. > Best, > Domenico > > > Domenico Gatti > Biochemistry & Mol. Biology > Wayne State University School of Medicine > 540 E. Canfield Avenue > Detroit, MI 48201 > Tel: 313-577-0620 or 313-993-4238 > Fax: 313-577-2765 > dg...@me... > > > > > > > Hi All -- > > Gernot is absolutely correct. I would also add that, after correcting the > issues Gernot raised below, you should also examine the sensitivity of your > results on "chgm spl0" vs. "chgm spl2" since charge discretization can > affect these types of calculations as well. > > Thanks, > > Nathan > > On Dec 10, 2009, at 9:31 AM, Gernot Kieseritzky wrote: > > > Hi! > > > > On Wed, 2009-12-09 at 17:49 +0000, J.Dziedzic wrote: > >> Hi! > >> > >> I am confused about the result of a very simple calculation when done > >> with APBS. Consider a trivial system of two point charges with unit > >> charges at a separation of 1 Bohr length, in vacuum. The Coulombic > >> energy of this system is exactly 1 Hartree, that is 2625.5 kJ/mole. > >> ... > >> ... APBS yields 31740 kJ/mole, which off by a factor of 12. Making the > >> grid finer only makes things worse, the results being: > >> > >> dime Energy (kJ/mole) > >> 65 1.209E04 > >> 129 2.222E04 > >> 193 3.174E04 > >> 257 4.058E04 > >> 289 4.539E04 > >> > >> which are all way off, by a factor of 5-20, from the correct value of > >> 2625.5 kJ/mole. > >> > >> I understand that normally one is interested in energy differences > >> between a system in vacuo and a solvated system, and any discretization > >> errors introduced are canceled if the grid is the same in both > >> calculations. Yet, with a system so trivial, without any dielectric > >> at all and, thus, without the arbitrariness of the cavity, what is the > >> underlying reason for the calculation being so off from the mark? > > > > Two words: grid artefact! Basically, the deviation is not due to > > numerical problems, rather the high energy values you observe are the > > result of the self-interaction of the grid points. That's why the > > deviation is increasing with higher resolution as the grid points are > > getting closer. What you have to do to get the total electrostatic > > energy of your system without self-energies: > > > > 1) Compute the Coulomb energy in the homogeneous continuum. > > > > 2) Compute the solvation energy of the same charge distribution using > > APBS. The grid artefact cancels as you calculate an energy difference > > here. This, of course, requires that you use the same grid setup in both > > APBS runs. > > > > 3) Add the values together. > > > > Best regards, > > Gernot Kieseritzky > > > > > ------------------------------------------------------------------------------ > Return on Information: > Google Enterprise Search pays you back > Get the facts. > http://p.sf.net/sfu/google-dev2dev > _______________________________________________ > apbs-users mailing list > apb...@li... > https://lists.sourceforge.net/lists/listinfo/apbs-users > -- Nathan Baker (http://bakergroup.wustl.edu) Associate Professor, Dept. of Biochemistry and Molecular Biophysics Director, Computational and Molecular Biophysics Graduate Program Center for Computational Biology, Washington University in St. Louis |