Re: [ESPResSo-users] Units of Temp and time step

[Axel Arnold]

On Friday 01 April 2011 11:15:24 Muhammad Anwar wrote:
> Hello,
> Dear User,
> I am using Kcal/mol for energy and Angstrom for distance, I want to set
> temperature of 300 K in Langevin thermostat and time step of 1 fs.  Will
> i write simply:
> 
> setmd time_step 1
> thermostat langevin 1.0 1.0

Hi,

there are 3 essential scales in a simulation, length, energy and mass, from 
which the rest of units can be derived. Masses in Espresso are by default 1,  
you need to compile in the feature MASS if you want to change that. If you 
keep it at 1, then your mass unit is the mass of any particle, and your time 
scale you get from

energy = mass*velocity^2 = mass*length^2/time^2

=> time = sqrt(mass/energy)*length.

As for the temperature, Espresso uses k_B=1. So, the scale for temperature is 
Kcal/mol/k_B. 

Axel

-- 
JP Dr. Axel Arnold
ICP, Universität Stuttgart
Pfaffenwaldring 27
70569 Stuttgart, Germany
Email: address@hidden
Tel: +49 711 685 67609