Application of quantum force computations for Raman spectroscopy and molecular dynamics

Electronic structure calculations have undergone incredible advancement in the past century. Using modern methods and supercomputing infrastructure we are now able to compute precise electron behavior in a variety of large and complex systems. However, these computations are only as good as their applications. To further these computations we consider two applications of efficient force calculations using first principles density functional theory. We compute the vibrational and Raman spectra for B-doped, P-doped, and B-P codoped Si nanocrystals using real-space pseudopotentials constructed within density functional theory. An experimental peak in the Raman spectra near 650 cm⁻¹ observed in codoped nanocrystals can be best explained by the presence of B-P bonds, which are located near the surface of the nanocrystal. We propose that the spectral details of this peak are related to quantum confinement and the breaking of local symmetry associated with the phonon modes involving dopant bonds. We also illustrate an improved method for calculation of nonlocal contributions to interatomic forces is used to perform molecular dynamics simulations. This method results from the real space density functional theory Hamiltonian utilizing a high order Gaussian integration scheme in real space. The efficacy of this method is demonstrated through molecular dynamics simulations of an O₂ molecule and a benzene molecule. Our method improves convergence of dynamic variables including stability and vibrational frequency