Attosecond charge migration in glycine and N-methylacetamide following sudden ionization: A TD-DMRG study

Attosecond charge migration following sudden ionization probes ionic-state coherence and multielectronic correlation and, therefore, requires multireference real-time methods able to treat large active spaces. Here, we apply the time-dependent density matrix renormalization group (TD-DMRG) with the time-dependent variational principle (TDVP) to study the early time charge migration in gas-phase glycine and N-methylacetamide (NMA) molecules within the fixed-nuclei, purely electronic regime. Target ionic states were constructed with matrix-product-state-based multireference configuration interaction on complete active space self-consistent field orbitals, and active orbitals were selected from a state-averaged one-electron reduced density matrix (1-RDM), yielding final active spaces of glycine (21e, 18o) and NMA (19e, 17o). The analysis of local partial charges, real-space hole densities, fixed-orbital hole occupations, and the autocorrelation function shows that the selected ionization channels follow distinct early time electronic-motion mechanisms. In glycine, the 10a', 11a', and 14a' channels sample three regimes: a correlation-driven inner-valence response with enhanced two-hole-one-particle satellite-state participation and multiorbital redistribution in 10a'; backbone-mediated charge redistribution in 11a'; and compact few-state terminal-group exchange in 14a'. The selected NMA 13a' channel, used as a benchmark for peptide-bond charge migration, is governed mainly by one-hole mixing and gives a regular back-and-forth oscillation across the amide region. The results identify the initially ionized orbital and the configurational composition of the ionic states as key factors controlling early time charge migration. TD-DMRG/TDVP therefore offers a practical ab initio route for simulating post-ionization electronic wave packets in relatively large active spaces.