  Second harmonic generation (SHG) is forbidden in centrosymmetric molecular
materials. However, a signal is frequently observed from interfaces where the
symmetry is broken. Whereas the effect can be phenomenologically accommodated,
an ab initio qualitative and quantitative description has remained elusive,
preventing the exploration of fascinating questions such as how deep below the
surface the second harmonic can still be generated. To answer such questions,
we present an ab initio multiscale approach to compute the total and
layer-dependent intensity of surface SHG from molecular crystals. The
microscopic origin of surface SHG is identified in layer-dependent models with
embedding partial charges combined with density functional theory. The models
show increasing symmetry-breaking distortions of the electron cloud around the
molecules as the surface layer is approached. The SHG at the molecular level is
determined using time-dependent density functional theory and then brought to
the scale of macroscopic films through a rigorous self-consistent multiple
scattering formalism capable of predicting the measurable optical intensities
of the generated second harmonic signal. We study crystalline molecular films
with centrosymmetric unit cells of 7,9-Dibromobenzo[h]quinolin-10-ol. The
intensity of the SHG at the surface layer is two orders of magnitude larger
than at the next layer below and three orders of magnitude larger than two
layers below. Besides providing fundamental understanding, our approach can be
used for designing and optimizing optical devices containing nonlinear
molecular materials, such as molecular laminates. We show that a relatively
basic Kretschmann-like setup can enhance the surface SHG of a crystalline film
of centrosymmetric molecular unit cells a thousand times.
