code: def evaluate_wavefunc(self, kpoint, band, r, spin=0):
          r
          v = self.Gpoints[kpoint] + self.kpoints[kpoint]
          u = np.dot(np.dot(v, self.b), r)
          c = self.coeffs[spin][kpoint][band] if self.spin == 2 else \
              self.coeffs[kpoint][band]
          return np.sum(np.dot(c, np.exp(1j * u, dtype=np.complex64))) / \
                 np.sqrt(self.vol)
docs: r"""
          Evaluates the wavefunction for a given position, r.
  
          The wavefunction is given by the k-point and band. It is evaluated
          at the given position by summing over the components. Formally,
  
          \psi_n^k (r) = \sum_{i=1}^N c_i^{n,k} \exp (i (k + G_i^{n,k}) \cdot r)
  
          where \psi_n^k is the wavefunction for the nth band at k-point k, N is
          the number of plane waves, c_i^{n,k} is the ith coefficient that
          corresponds to the nth band and k-point k, and G_i^{n,k} is the ith
          G-point corresponding to k-point k.
  
          NOTE: This function is very slow; a discrete fourier transform is the
          preferred method of evaluation (see Wavecar.fft_mesh).
  
          Args:
              kpoint (int): the index of the kpoint where the wavefunction
                              will be evaluated
              band (int): the index of the band where the wavefunction will be
                              evaluated
              r (np.array): the position where the wavefunction will be evaluated
              spin (int):  spin index for the desired wavefunction (only for
                              ISPIN = 2, default = 0)
          Returns:
              a complex value corresponding to the evaluation of the wavefunction