Multireference Theories of Electron Correlation Based on the Driven Similarity Renormalization Group

The driven similarity renormalization group (DSRG) provides an alternative way to address the intruder state problem in quantum chemistry. In this review, we discuss recent developments of multireference methods based on the DSRG. We provide a pedagogical introduction to the DSRG and its various extensions and discuss its formal properties in great detail. In addition, we report several illustrative applications of the DSRG to molecular systems.

Nucleon–anti-nucleon intruder state of Dirac equation for nucleon in deep scalar potential well

We solve the Dirac radial equation for a nucleon in a scalar Woods–Saxon potential well of depth [Formula: see text] and radius [Formula: see text]. A sequence of values for the depth and radius are considered. For shallow potentials with [Formula: see text] the wave functions for the positive-energy states [Formula: see text] are dominated by their nucleon component [Formula: see text]. But for deeper potentials with [Formula: see text] the [Formula: see text]s begin to have dominant anti-nucleon component [Formula: see text]. In particular, a special intruder state enters with wave function [Formula: see text] and energy [Formula: see text]. We have considered several [Formula: see text] values between 2 and 8[Formula: see text]fm. For [Formula: see text] and the above [Formula: see text] values, [Formula: see text] is the only bound positive-energy state and has its [Formula: see text] closely equal to [Formula: see text], both having a narrow wave packet shape centered around [Formula: see text]. The [Formula: see text] of this state is practically independent of [Formula: see text] for the above [Formula: see text] range and obeys closely the relation [Formula: see text].