Potential term to restrain the volume associated with the gyration tensor. This is used as an alternative to a radius of gyration resrtaint- to allow elongated structures. The target value is set in .create_GyrPot based on values observed in the PDB. constructor: GyrPot(instanceName, selection ) instanceName is a user-specified identifier selection is a .AtomSel object specifying the atoms to include when calculating the gyration tensor. methods calcEnergy() - calc energy, returns the energy value. calcEnergyAndDerivs(derivs) - calc energy, derivs, returns the energy value. rms() - return the magnitude of violation of this term -- abs(diff). scale() - scale factor for this energy term. selection() - return the atom selection specified in the constructor. info() - current info about the state of this instance qc() - The calculated centroid of selection(). gyrTensor() - The calculated gyration tensor. Rg() - The calculated radius of gyration ( Tr G ) volume() - The calculated gyration volume. eigenPairs() - The three principal moments and directions of the gyration tensor. The following parameters can be set [defaults in square brackets] volTarget - The target gyration volume. volRange - The +/- range about the target volume about which to evaluate zero energy (see below). the above quantities may be retrieved using the member function form quantity(), while they are set using the form setQuantity(value). The gyration tensor is defined as G = 1/N sum_i (q_i - qc ) X (q_i - qc) where N is the number of selected atoms, q_i is atom i's position and qc is the centroid of the selected atoms. The ``X'' denotes outer product. The gyration volume is Vg = 4/3 pi sqrt(det G) The energy function is defined as V = scale * diff^2 where Vg - targetVol - rangeVol if Vg>targetVol+rangeVol diff = targetVol - rangeVol - Vg if Vg