Example: Interchain Interaction Energy

The following example shows how to restrict the energy calculation to the interaction energy between two chains with segment identifiers ``a" and ``b." Only the energies and forces between the selected atoms are computed.

CONStraints INTEraction 
    ( segid a ) ( segid b ) 
END

This example does not include the intrachain energy. To include the intrachain energy, one should specify the following:

CONStraints INTEraction 
    ( segid a or segid b ) ( segid a or segid b ) 
END

The following example modifies the weight of the inter- and intrachain interactions in the Hamiltonian, applying a weight of 0.5 to the bond energies and forces and a factor of 0.75 to the other energy terms :

CONStraints INTEraction 
   ( segid a or segid b ) ( segid a or segid b ) WEIGhts * 0.75 bonds 0.5 END 
END

This example includes the intrasegment bond energy and the overall intersegment energy but excludes the intrasegment angle and dihedral, improper, and nonbonded terms from the Hamiltonian :

CONStraints
    INTEraction ( segid a ) ( segid b ) WEIGhts * 1. END 
    INTEraction ( segid a ) ( segid a ) WEIGhts * 0. bonds 1. END 
    INTEraction ( segid b ) ( segid b ) WEIGhts * 0. bonds 1. END 
END

Three double selections are set up, each with a different weighting scheme. The Hamiltonian is the sum of the three partial Hamiltonians. The result is to turn off the intrasegment interactions other than the bonds.