calculating refractive indices



 Dear Netters,
         Does anyone know how to calculate refractive index of a molecule?
 I know it can be estimated using molar refractivities for each bond , but I
 can't find a table of these that includes a C=N bond.  Is there a better
 way to calculate this property, maybe using computationally obtained values
 for polarizability?
                                                 Thanks in advance,
                                                 Brad Kaufman
                                                 Bryn Mawr College
                                                 bkaufman : at : cc.brynmawr.edu