A semiclassical approach is developed for nonequilibrium quantum transport in molecular junctions. Following the early work of Miller and White [J. Chem. Phys. 84, 5059 (1986)], the many-electron Hamiltonian in second quantization is mapped onto a classical model that preserves the fermionic character of electrons. The resulting classical electronic Hamiltonian allows for real-time molecular dynamics simulations of the many-body problem from an uncorrelated initial state to the steady state. Comparisons with exact results generated for the resonant level model reveal that a semiclassical treatment of transport provides a quantitative description of the dynamics at all relevant timescales for a wide range of bias and gate potentials, and for different temperatures. The approach opens a door to treating nontrivial quantum transport problems that remain far from the reach of fully quantum methodologies.

We apply the recently proposed quasi-classical approach for a second quantized many-electron Hamiltonian in Cartesian coordinates [B. Li and W. H. Miller, J. Chem. Phys. 137, 154107 (2012)] to correlated nonequilibrium quantum transport. The approach provides accurate results for the resonant level model for a wide range of temperatures, bias, and gate voltages, correcting the flaws of our recently proposed mapping using action-angle variables. When electron-electron interactions are included, a Gaussian function scheme is required to map the two-electron integrals, leading to quantitative results for the Anderson impurity model. In particular, we show that the current mapping is capable of capturing quantitatively the Coulomb blockade effect and the temperature dependence of the current below and above the blockade.

The description of the dynamics of correlated electrons in quantum impurity models is typically described within the nonequilibrium Green function formalism combined with a suitable approximation. One common approach is based on the equation-of-motion technique often used to describe different regimes of the dynamic response. Here, we show that this approach may violate certain symmetry relations that must be fulfilled by the definition of the Green functions. These broken symmetries can lead to unphysical behavior. To circumvent this pathological shortcoming of the equation-of-motion approach we provide a scheme to restore basic symmetry relations. Illustrations are given for the Anderson and double Anderson impurity models.

Callous-unemotional (CU) traits correlate with the severity and prognosis of conduct disorder in youth. The neuropeptide oxytocin (OT) has been linked to prosocial behaviors, including empathy and collaboration with others. This study discusses a possible role for OT in the biology of delinquent behavior. We hypothesized that in delinquent youth OT secretion will correlate with the severity of conduct problems and specifically with the level of CU traits. The study group included 67 male adolescents (mean age 16.2 years) undergoing residential treatment, previously assessed by an open clinical interview and history for the psychiatric diagnosis. Staff based Inventory of Callous-Unemotional traits for psychopathy and Strength and Difficulties Questionnaire were administered, and patients' medical and social personal files were systematically coded for previous history of antisocial acts using the Brown-Goodwin Questionnaire. Salivary OT was assayed by ELISA. Salivary OT levels were inversely correlated with conduct problems severity on Strength and Difficulties Questionnaire (r = -0.27; p ≤ 0.01). Recorded history of antisocial acts did not correlate with current OT levels. Odds ratio (OR) for significant CU traits among subjects with conduct problems was increased in low-OT (OR = 14, p ≤ 0.05) but not in high-OT subjects (OR = 6, p ≥ 0.05). Children with conduct problems and low levels of salivary OT are at risk for significant CU traits. These results suggest a possible role for salivary OT as a biomarker for CU traits and conduct problems severity.