import numpy as np

def potential(r):
    distance = np.sqrt(np.dot(r,r))
    assert (distance > 0)
    return -1. / distance

def psi(a, r):
    return np.exp(-a*np.sqrt(np.dot(r,r)))

def kinetic(a,r):
    distance = np.sqrt(np.dot(r,r))
    assert (distance > 0.)
    return -0.5 * (a**2 - (2.*a)/distance)

def e_loc(a,r):
    return kinetic(a,r) + potential(r)

def drift(a,r):
   ar_inv = -a/np.sqrt(np.dot(r,r))
   return r * ar_inv