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Noble-Gas Chemistry
Published in Leonid Khriachtchev, Physics and Chemistry at Low Temperatures, 2019
Wojciech Grochala, Leonid Khriachtchev, Markku Rasanen
As mentioned earlier, the direct formation of HNgY molecules upon photolysis of the HY precursor is observed only in special cases. This limitation is due to the fact that the HNgY molecules possess very strong dissociative electronic absorptions where the photosensitivity originates from strong charge-transfer absorptions onto a repulsive excited electronic state.79−81 Correlation of the IR spectra with the broad UV absorptions allows identification of the individual UV spectra of different HNgY molecules. The transition dipoles involved exceed 7 D as predicted by multireference configuration interaction calculations.79 The experimental estimates agree with the theoretical predictions.55,81
D)
Published in Ram K. Gupta, Sanjay R. Mishra, Tuan Anh Nguyen, Fundamentals of Low Dimensional Magnets, 2023
In spite of the success of the CP-DFT method, one must admit that the single reference character of the DFT is unable to describe the multireference character of the spin eigenfunctions involved in the determination of ZFS, which necessitates employment of the multireference methods in the evaluation of D. The complete active space self-consistent field has emerged as one of the central methods for dealing with the spin properties of magnetic molecules. Generally, WFT-based methods compute ZFS in two steps. At first, the electronic states are determined from state-averaged multi-configurational SCF calculations using the CASSCF scheme for selecting relevant electronic configurations. The resulting configuration provides a good representation of the static electron correlation, though this is not accurate enough for evaluating spin Hamiltonian parameters. Hence, dynamic correlation is added using perturbative methods, which consider the effects of the configurations external to the complete active space, and the ground state energy is refined through perturbation of second order. In this regard, the most popular methods have been the complete active space perturbation theory and the N-electron valence perturbation theory. Both methods include all single and double excitations involving at least one inactive or one virtual orbital. Another reliable method to increase the accuracy of the computed spin Hamiltonian parameters is the multireference configuration interaction (MRCI) method. However, numerous numbers of single and double excitations, as should be inevitably considered in the MRCI method (MRCI-SD), are not computationally feasible, and thus it becomes necessary to truncate the MRCI-SD space. In this regard, this is to recall that CASSCF computation is based on the partition of the MO into three subspaces: the inactive orbitals (which remain doubly occupied in all the configurations), the active orbitals (singly or doubly occupied), and the virtual orbitals (which remain unoccupied in all the configurations). An annihilation of an electron (one hole, 1h) in the inactive set and the creation of an electron (one particle, 1p) in a virtual set of orbitals correspond to a degree of freedom. For example, single-electron excitation from the inactive to active set (1h) or from active to virtual set (1p) corresponds to one degree of freedom, whereas double electron excitation (2h/2p) within the same set defines two degrees of freedom. Similarly, double electronic excitation from inactive to virtual (2h-2p) is associated with four degrees of freedom and so on. Truncation of the MRCI-SD space leads to different methods such as DDCI3, DDCI2, etc. where the suffixed number indicates the maximum number of degrees of freedom in the method concerned. It has been shown that the 2h-1p and 1h-2p excitations play a crucial role in the calculation of the energy difference between different spin states.[63]
Calculations of atomisation energy and singlet–triplet gap with iterative multireference configuration interaction
Published in Molecular Physics, 2022
Jia-Qi Fan, Wen-Yan Zhang, Qing Ren, Feiwu Chen
Among various electronic structure theories [22–36], the multireference configuration interaction (MRCI) is one of the most accurate and reliable methods [22,24,25,30,36]. Recently, Zhang and Chen proposed an iterative multireference configuration interaction (IMRCI) [36], which is to some extent similar to the iCI method proposed by Liu et al. [24,25,30]. iCI method can be considered as a combination of the effective Hamiltonian theory and the contracted configuration interaction, in which a 3Np × 3Np matrix is constructed and diagonalised in each iteration until a given energy criterion is reached [24,25,30]. As for IMRCI [36], the multireference configuration functions in the reference space of a given size are updated iteratively. The important configurations with the larger absolute variation coefficients in each configuration interaction calculation are selected for the multireference space until the configuration functions in the multireference space remain unchanged. The IMRCI errors relative to the full configuration interaction (FCI) results are at the order of magnitude of 10−5 hartree within just 2–4 iterations. Further IMRCI can also be used to find the main electron configurations on the potential energy surfaces. The present work aims to apply the IMRCI method to calculating the atomisation energies and singlet–triplet separations, and to test the effect of the frozenness of the highest unoccupied molecular orbital (HUMO) on the IMRCI results.
Density functional and ab initio study of samarium dihalides, SmX2 (X = I, Br, and Cl)
Published in Molecular Physics, 2019
Jiwon Moon, Heehyun Baek, Joonghan Kim
The state-average complete active space self-consistent field (SA-CASSCF) method [43] was used to calculate the septet spin states as well as other lower spin states such as quintet, triplet, and singlet of SmI2. The active orbitals contain seven f orbitals of Sm. Hereafter, it is denoted as CAS(6,7). The active orbitals are shown in Figure S1 in the Supplemental Material. The RECP and basis sets in CAS(6,7) calculations are the same as those of DFT, MP2, and CCSD(T) calculations. The multireference configuration interaction with Davidson correction (MRCI + Q) method [44-46] was used to consider the dynamic electron correlation effect. However, no frozen core as in the CCSD(T) calculations could not apply in the MRCI + Q calculations due to computational cost. Thus, we examined the effect of frozen core size in the MRCI + Q calculations; the results are summarised in Table S1 in the Supplemental Material. No effect of frozen core size in the relative energies of septet spin states of SmI2 was observed. It is noted that the results of SA-CAS(6,7) are similar to those of MRCI + Q (Table S1 in the Supplemental Material). Therefore, the SA-CAS(6,7) method was used to calculate the relative energies of lower spin states of SmI2. All SA-CAS(6,7) and MRCI + Q calculations were performed using the Molpro2015 programme [47].
Spectroscopic properties of the molecular ions BeX+ (X=Na, K, Rb): forming cold molecular ions from an ion–atom mixture by stimulated Raman adiabatic process
Published in Molecular Physics, 2018
Hela Ladjimi, Dibyendu Sardar, Mohamed Farjallah, Nisrin Alharzali, Somnath Naskar, Rym Mlika, Hamid Berriche, Bimalendu Deb
This study has three parts. In the Part-I, we calculate adiabatic potential energy curves for (BeNa)+, (BeK)+ and (BeRb)+ molecular ionic systems by ab initio approach involving a non-empirical pseudo-potential for the X+(X=Na, K, Rb) and Be2+ cores. Here, core-valence correlation is accounted in an empirical form of core-polarisation potentials and multireference configuration interaction technique is employed. In the potential energy plot, short range part is obtained by our ab initio calculation and long-range potential is obtained by the sum of the dispersion terms described by −1/2(C4/r4 + C6/r6). We calculate spectroscopic constants and transition dipole moment for these systems. In Part-II of this work, we investigate elastic hetero nuclear cold collisions for a wide range of energies ranging from 0.01 μK to 1 K between alkaline-earth ion Be+ and neutral alkali atom X (X=Na, K, Rb) of each BeX+ system in their first excited electronic state. At low energy regime, hetero-nuclear ion–atom collision is dominated by elastic scattering process and charge transfer process is suppressed.