def get_forces(self, a):
        """Calculate atomic forces."""
        f = np.zeros( [ len(a), 3 ], dtype=float )
        for c in self.calcs:
            f += c.get_forces(a)
        return f
def get_forces ( self , a ) : f = np . zeros ( [ len ( a ) , 3 ] , dtype = float ) for c in self . calcs : f += c . get_forces ( a ) return f
Calculate atomic forces.