Forcefield calls

As is the case with any molecular simulation code, Music's core also is the forcefield routines. However, Music has a unique ability to store forcefield interaction results to facilitate flexible use of the information without costly re-evaluations. The storage is a separate code object that is initialized at a user-selected level of detail. Greater detail means greater memory requirements and usually slower run-times, but allows extraction of more fundamental quantities.

Here it is explained how one can make forcefield calls from any part of the code. This documentation is designed for developers. Normal users will not need this information. The subroutines and functions called here are from the source code of Music. Look at their headers for more help.

Some useful terminology:

'subset' - A subset of the system is portion of the system defined by its species, molecule and atom number. It can be defined by an array of 3 integers and may refer to any degree of detail. Here's some examples:

• 0 = full system subset = (/0,0,0/)
• 1 = species level subset = (/a,0,0/)
• 2 = molecule level subset = (/a,b,0/)
• 3 = atom level subset = (/a,b,c/)

'subset-subset' interactions are then interactions between two defined pieces of the system. An example would be subset1=(/1,0,0/) (species 1) interacting with subset1=(/2,1,5/) (5th atom of 1st molecule of species 2).

'storage detail' or 'storage detail level' refers to the amount of interactions detail that is stored. 'Detail' refers to levels of the system-species-molecule-atom hierarchy. Here are the available settings:

• SPC - will store all species-species interactions, nothing more detailed
• MOL - will store all molecule-molecule as well as all species-species interactions.
• ATM - will store all atom-atom, molecule-molecule and species-species interactions.

In any case, if the number of derivatives stored in the storage structure is greater than zero, then forces on each atom will be stored. MOST IMPORTANT: if it has been stored, then you can request it from the storage at any point by using the interact_extract routine!!