Semiclassical Molecular Dynamics for Condensed Phase NMR Spectroscopy

We use the Meyer-Miller formalism and the Caldeira-Leggett model with its associated and discretized spectral density formalism, to explore the impact of harmonic oscillator baths on the spectral line shapes of single- and two-spinI=12 NMR spin systems. The complex coefficients associated with the spin degrees of freedom are treated as classical position, qn, and momentum, pn, variables to investigate the coupling to the classical harmonic bath oscillators. The expressions derived for the motion of the density matrix elements associated with the observable NMR signal and modulated by the bath oscillators are used in the numerical simulations of NMR spectra. Unlike existing relaxation theories such as the Redfield theory, the method is not limited to short, bath correlation times. This is considered a strength of the method, and simulations have been performed at bath correlation times considered to be on the limit or beyond regimes where Redfield theory usually is applicable. Simulated line shapes are considerably broader than those predicted by Redfield theory. These results are discussed in terms of inclusion of nonsecular bath frequencies and how properties of the bath oscillators are defined.