LARGE LOCAL-FIELD EFFECTS IN THE 2ND-HARMONIC SUSCEPTIBILITY OF CRYSTALLINE UREA

The linear-optical response and the nonlinear susceptibility for secon d-harmonic generation are calculated for crystalline urea within band theory using the Kohn-Sham local-density approximation, implemented wi th pseudopotentials and plane waves. In the low-frequency limit, the s tatic dielectric constants are found to be 2.03 (ordinary) and 2.14 (e xtraordinary), vs 2.17 and 2.49, respectively, for the experimental va lues. For second-harmonic generation, Kleinmann symmetry is invoked to consider a single independent tensor component, d14=d36. The calculat ed value is 1.1 pm/V, compared to an experimental value of 1.2+/-0.1 p m/V for d36. Without local-field corrections, d would have been 2.1 pm /V. Hence, nonlinear local-field corrections are a key element of this calculation.