If OCTTargetOperator = oct_tg_classical, the you must supply this block.
It should contain a string (e.g. "(q[1,1]-q[1,2])*p[2,1]") with a mathematical
expression in terms of two arrays, q and p, that represent the position and momenta
of the classical variables. The first index runs through the various classical particles,
and the second index runs through the spatial dimensions.
In principle, the block only contains one entry (string). However, if the expression is very
long, you can split it into various lines (one column each) that will be concatenated.
The QOCT algorithm will attempt to maximize this expression, at the end of the propagation.