[gmx-users] LJ Switch function in the CVS VERSION 3.3_beta_20050202

[Erik Lindahl]

Hi,

David is right - the switching function was changed to 5th order to  
make both potential and force smooth. However, I too recommend  
against using switched interactions due to the artificial _increase_  
in force when switching.

Cheers,

Erik

On Sep 9, 2005, at 8:19 AM, David wrote:

> On Thu, 2005-09-08 at 18:35 -0400, Xuhui Huang wrote:
>
>> Dear Gromacs users,
>>     We noticed that the LJ switch function in the CVS Version
>> 3.3_beta_20050202 gives the wrong energy. We tested this with the
>> water system in the tutor. With the LJ switch function, the CVS
>> version gives the LJ(SR) energy of  9.28495e+02 kJ/mol, however, the
>> Gromacs 3.1.4 gives the energy of 2.00385e+03 kJ/mol. With the LJ  
>> cut-
>> off, both version gives the energy about 2.0e+03 kJ/mol.
>>    Does anyone notice this problem before?
>>
>
> Somehow I recall that the functional for for switch changed. Since  
> it is
> useless anyway (except for maybe an Argon system), don't wast any time
> on it.
>
>>
>>    The following is our parameters about the LJ switch part in
>> the .mdp part,
>>
>>    ; Method for doing Van der Waals
>>    vdw-type                 = Switch ;Cut-off
>>    ; cut-off lengths
>>    rvdw-switch              = 0.75
>>    rvdw                     = 0.85
>>
>>
>>
>> Thanks
>> Xuhui
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> -- 
> David.
> ______________________________________________________________________ 
> __
> David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
> Dept. of Cell and Molecular Biology, Uppsala University.
> Husargatan 3, Box 596,          75124 Uppsala, Sweden
> phone:  46 18 471 4205          fax: 46 18 511 755
> spoel at xray.bmc.uu.se    spoel at gromacs.org   http://xray.bmc.uu.se/ 
> ~spoel
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 
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