# chgm

Specify the method by which the biomolecular point charges (i.e., Dirac delta functions) by which charges are mapped to the grid for a multigrid (mg-manual, mg-auto, mg-para) Poisson-Boltzmann calculation. As we are attempting to model delta functions, the support (domain) of these discretized charge distributions is always strongly dependent on the grid spacing. The syntax is:

    chgm {flag}

`flag` is a text string that specifies the type of discretization:

`spl0`
  Traditional trilinear interpolation (linear splines). The charge is mapped onto the nearest-neighbor grid points. Resulting potentials are very sensitive to grid spacing, length, and position.
`spl2`
  Cubic B-spline discretization. The charge is mapped onto the nearest- and next-nearest-neighbor grid points. Resulting potentials are somewhat less sensitive (than `spl0`) to grid spacing, length, and position.
`spl4`
  Quintic B-spline discretization. Similar to `spl2`, except the charge/multipole is additionally mapped to include next-next-nearest neighbors (125 grid points receive charge density).