Control of force output

The total force acting on an atom in an MD simulation is composed of several contributions, e.g., of forces arising from bonding terms, angle terms or from electrostatic interactions. In the free format section of EGO one can specify that the sum of such a force contribution is written to file for later analysis. For example, if one likes to know the force arising from bonding contributions acting on a certain atom (or a set of atoms) one can specify the keyword BOND and a selection string:
BOND A5
Then, the sum of forces arising from bonding terms acting on atom 5 will be printed to the file bond_force.out located in the output directory. The file format is ASCII and easy to understand. The general format to control force output is
<FORCETYPE> <atom selection string>
Possible keywords for <FORCETYPE> are:
BOND     : Write bond forces to file bond_force.out
ANGLE    : Write angle forces to file angle_force.out
DIHE     : Write dihedral forces to file dihe_force.out
IMPR     : Write improper forces to file impr_force.out
ELEC     : Write electrostatic forces to file elec_force.out
VDW      : Write van der Waals forces to file vdw_force.out
RESTR    : Write restraint forces (arising from the SBOUND region) to file restr_force.out
ELECVDW  : Write the sum of electrostatic and van der Waals forces to file elecvdw_force.out
INTERN   : Write the sum of BOND, ANGLE, DIHEDRAL and IMPROPER to the file intern_force.out
TOTAL    : Write the total forces to the file total_force.out
For further information about the format of the atom selection string see Section 4.3.
  
In the free format section there is also the possibility to control the output of individual forces acting between atoms. For example, if one likes to know the bond force between atom 5 and 6 one has to specify
PICKBOND 5 6
Then, the force between the two atoms is printed to file bond_pick.out together with distance information. The file format is again ASCII. Note, that there will be only a force between atom 5 and 6 if there is a covalent bond specified in the structure file of your model system.

Other forces, which can be `picked' like that, are the electrostatic and van der Waals forces, e.g., by specifying
PICKELEC 10 203
PICKVDW 10 203
However, due to the FAMUSAMM algorithm these forces are only explictily calculated if they are closer than about 9Å. For more distant pairs of atoms these forces are approximated by a multipole scheme which calculates forces between groups of atoms rather than pairs of atoms. Thus, zero is printed to the output file if one selects two atoms more distant than about 9Å.

If one likes to pick angle forces between a tripplet of atoms one, e.g., specifies,
PICKANGLE 4 6 8
Dihedral and improper forces can be picked by specifying
PICKDIHE 4 6 8 9 or
PICKIMPR 4 6 8 9
  
There are two further keywords (START, STRIDE and IDSTRING) which are important for the control of force output. The keyword START specifies at which integration step EGO begins to write out the forces to file. The keyword STRIDE controls the frequency of writing out the forces. With the keyword IDSTRING one can set a string which modifies the name of the force output files. Example:
START 500
STRIDE 10
IDSTRING simulation1_
INTERN A5
ELEC A5
After integration step 500 and every 10th integration step the corresponding forces are written to file. The files will be named simulation1_intern_force.out and simulation1_elec_force.out.   
Important note: Writing out forces to file considerably reduces the speed of your calculation and takes a lot of disk space if you select a large set of atoms.