For a system without spin–orbit coupling, the (i) nuclear plus electronic linear momentum and (ii) nuclear plus orbital electronic angular momentum are good quantum numbers. Thus, when a molecular system undergoes a nonadiabatic transition, there should be no change in the total linear or angular momentum. Now, the standard surface hopping algorithm ignores the electronic momentum and indirectly equates the momentum of the nuclear degrees of freedom to the total momentum. However, even with this simplification, the algorithm still does not conserve either the nuclear linear or the nuclear angular momenta. Here, we show that one way to address these failures is to dress the derivative couplings (i.e., the hopping directions) in two ways: (i) we disallow changes in the nuclear linear momentum by working in a translating basis (which is well known and leads to electron translation factors) and (ii) we disallow changes in the nuclear angular momentum by working in a basis that rotates around the center of mass [which is not wellknown and leads to a novel, rotationally removable component of the derivative coupling that we will call electron rotation factors below, cf. Eq. (96)]. The present findings should be helpful in the short term as far as interpreting surface hopping calculations for singlet systems (without spin) and then developing the new surface hopping algorithm in the long term for systems where one cannot ignore the electronic orbital and/or spin angular momentum.
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Within the context of fewestswitch surface hopping (FSSH) dynamics, one often wishes to remove the angular component of the derivative coupling between states J and K. In a previous set of papers, Shu et al. [J. Phys. Chem. Lett. 11, 1135–1140 (2020)] posited one approach for such a removal based on direct projection, while we isolated a second approach by constructing and differentiating a rotationally invariant basis. Unfortunately, neither approach was able to demonstrate a oneelectron operatorÔ whose matrix element JÔK was the angular component of the derivative coupling. Here, we show that a oneelectron operator can, in fact, be constructed efficiently in a semilocal fashion. The present results yield physical insight into designing new surface hopping algorithms and are of immediate use for FSSH calculations.
more » « less Award ID(s):
 2102402
 NSFPAR ID:
 10520187
 Publisher / Repository:
 jcp
 Date Published:
 Journal Name:
 The Journal of Chemical Physics
 Volume:
 160
 Issue:
 12
 ISSN:
 00219606
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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