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Magnetic molecules and magnetic logic
Published in Guo-ping Zhang, Georg Lefkidis, Mitsuko Murakami, Wolfgang Hübner, Tomas F. George, Introduction to Ultrafast Phenomena from Femtosecond Magnetism to high-harmonic Generation, 2020
Guo-ping Zhang, Georg Lefkidis, Mitsuko Murakami, Wolfgang Hübner, Tomas F. George
We first calculate the lowest many-body electronic state in Ni2 as a function of the interatomic distance using high-level n-electron valence-state perturbation theory [NEVPT2, see Angeli et al. (2001)] as well as the equation-of-motion coupled cluster (EOM-CCSD) method [Krylov (2008)]. Our results are shown in Fig. 8.4. One main observation is that the equilibrium distance of the electronic ground state very much depends on the basis set and correlation method used: The NEVPT2 method yields 2.2 ˚A [Chaudhuri et al. (2015), Fig. 8.4], which is very close to the experimental value of 2.15 ˚A, while SAC-CI and EOM-CCSD computations, depending on the exact basis set used, result in slightly longer interatomic distances, as long as 2.4 ˚A [Dong et al. (2013); Chaudhuri et al. (2014, 2015), see also Fig. 8.4]. Subsequently we derive successful spin-flip scenarios at several distances. Because Ni2 is highly symmetric, the spin density for all low-lying electronic states is equidistributed among both Ni atoms; hence we derive global and not local spin-flips. For each interatomic distance, we find a different optimized laser pulse.
Photo-induced primary processes of trans-[Co(acac)2(N3)(py)] in liquid solution studied by femtosecond vibrational and electronic spectroscopies
Published in Molecular Physics, 2021
Tobias Unruh, Luis I. Domenianni, Peter Vöhringer
Trans-[1] is a neutral diamagnetic complex of trivalent cobalt and as such, it has a singlet (S = 0) ground state with a d6 configuration at the metal. The linear electronic absorption spectrum of the complex in liquid dichloromethane solution at room temperature is displayed in Figure 2(a). The spectrum can be divided formally into two spectral areas; the region of the metal-centered d-d transitions above wavelengths of about 400 nm and the region dominated by charge-transfer transitions below 400 nm. The electronic structure of low-spin cobalt(III) complexes with quasi-tetragonally symmetric ligand sphere has recently been scrutinised with complete active space self-consistent field (CASSCF) calculations including n-electron valence state perturbation theory (NEVPT2) [30].
Improved stochastic multireference perturbation theory for correlated systems with large active spaces
Published in Molecular Physics, 2020
James J. Halson, Robert J. Anderson, George H. Booth
Our initial work applied this approach to both 2nd-order Complete Active Space Perturbation Theory (CASPT2) and 2nd-order N-Electron Valence state Perturbation Theory (NEVPT2) [11]. However, it was shown that CASPT2 calculations based on FCIQMC observables are prone to numerical instability [14]. CASPT2 requires inverting the inherently noisy matrices estimated within the FCIQMC dynamic. This non-linear operation amplifies any stochastic error in these quantities, and therefore the resulting CASPT2 correction is very sensitive to any noise in the FCIQMC wavefunction. In contrast, strongly contracted NEVPT2 avoids many of these difficulties of inverting stochastically derived matrices, as well as difficulties with intruder states [15]. Because of this, we found NEVPT2 to be stable and performed well, converging rapidly with walker number to the deterministic results, and allowing straightforward extension to active space sizes beyond those traditionally accessible. Despite this success, for larger active spaces, the sampling of high-rank tensors in the active space in FCIQMC-NEVPT2 can still become a bottleneck of the computation and represents a significant fraction of the time on top of the original active space FCIQMC algorithm. It is this challenge which we tackle in this work, developing and applying an improved sampling scheme for the FCIQMC-NEVPT2 method using both a stochastic CASCI, and also coupled with a self-consistent orbital optimisation obtained via a fully stochastic FCIQMC-CASSCF optimisation [16].