Tran Nguyen Lan

Complete active space second-order perturbation theory with cumulant approximation for extended active-space wavefunction from density matrix renormalization group

We report an extension of our previous development that incorporated quantum-chemical density matrix renormalization group (DMRG) into the complete active space second-order perturbation theory (CASPT2) [Y. Kurashige and T. Yanai, J. Chem. Phys. 135, 094104 (2011)]. In the previous study, the combined theory, referred to as DMRG-CASPT2, was built upon the use of pseudo-canonical molecular orbitals (PCMOs) for one-electron basis. Within the PCMO basis, the construction of the four-particle reduced density matrix (4-RDM) using DMRG can be greatly facilitated because of simplicity in the multiplication of 4-RDM and diagonal Fock matrix in the CASPT2 equation. In this work, we develop an approach to use more suited orbital basis in DMRG-CASPT2 calculations, e.g., localized molecular orbitals, in order to extend the domain of applicability. Because the multiplication of 4-RDM and generalized Fock matrix is no longer simple in general orbitals, an approximation is made to it using the cumulant reconstruction neglecting higher-particle cumulants. Also, we present the details of the algorithm to compute 3-RDM of the DMRG wavefunction as an extension of the 2-RDM algorithm of Zgid et al. [J. Chem. Phys. 128, 144115 (2008)] and Chan et al. [J. Chem. Phys. 128, 144117 (2008)]. The performance of the extended DMRG-CASPT2 approach was examined for large-scale multireference systems, such as low-lying excited states of long-chain polyenes and isomerization potential of {[Cu(NH3)3]2O2}(2+).

Toward Reliable Prediction of Hyperfine Coupling Constants Using Ab Initio Density Matrix Renormalization Group Method: Diatomic (2)Σ and Vinyl Radicals as Test Cases.

The density matrix renormalization group (DMRG) method is used in conjunction with the complete active space (CAS) procedure, the CAS configuration interaction (CASCI), and the CAS self-consistent field (CASSCF) to evaluate hyperfine coupling constants (HFCCs) for a series of diatomic (2)Σ radicals (BO, CO(+), CN, and AlO) and vinyl (C2H3) radical. The electron correlation effects on the computed HFCC values were systematically investigated using various levels of active space, which were increasingly extended from single valence space to large-size model space entailing double valence and at least single polarization shells. In addition, the core correlation was treated by including the core orbitals in active space. Reasonably accurate results were obtained by the DMRG-CASSCF method involving orbital optimization, while DMRG-CASCI calculations with Hartree-Fock orbitals provided poor agreement of the HFCCs with the experimental values. To achieve further insights into the accuracy of HFCC calculations, the orbital contributions to the total spin density were analyzed at a given nucleus, which is directly related to the FC term and is numerically sensitive to the level of correlation treatment and basis sets. The convergence of calculated HFCCs with an increasing number of renormalized states was also assessed. This work serves as the first study on the performance of the ab initio DMRG method for HFCC prediction.

Scalar relativistic calculations of hyperfine coupling constants using ab initio density matrix renormalization group method in combination with third-order Douglas-Kroll-Hess transformation: case studies on 4d transition metals.

We have developed a new computational scheme for high-accuracy prediction of the isotropic hyperfine coupling constant (HFCC) of heavy molecules, accounting for the high-level electron correlation effects, as well as the scalar-relativistic effects. For electron correlation, we employed the ab initio density matrix renormalization group (DMRG) method in conjunction with a complete active space model. The orbital-optimization procedure was employed to obtain the optimized orbitals required for accurately determining the isotropic HFCC. For the scalar-relativistic effects, we initially derived and implemented the Douglas-Kroll-Hess (DKH) hyperfine coupling operators up to the third order (DKH3) by using the direct transformation scheme. A set of 4d transition-metal radicals consisting of Ag atom, PdH, and RhH2 were chosen as test cases. Good agreement between the isotropic HFCC values obtained from DMRG/DKH3 and experiment was archived. Because there are no available gas-phase values for PdH and RhH2 radicals in the literature, the results from the present high-level theory may serve as benchmark data.

Molecular g-tensors from analytical response theory and quasi-degenerate perturbation theory in the framework of complete active space self-consistent field method

The molecular g-tensor is an important spectroscopic parameter provided by electron para magnetic resonance (EPR) measurement and often needs to be interpreted using computational methods. Here, we present two new implementations based on the first-order and second-order perturbation theories to calculate the g-tensors within the complete-active space self-consistent field (CASSCF) wave function model. In the first-order method, the quasi-degenerate perturbation theory (QDPT) is employed for constructing relativistic CASSCF states perturbed with the spin–orbit coupling operator, which is described effectively in one-electron form with the flexible nuclear screening spin–orbit approximation introduced recently by us. The second-order method is a newly reported approach built upon the linear response theory which accounts for the perturbation with respect to external magnetic field. It is implemented with the coupled–perturbed CASSCF (CP-CASSCF) approach, which provides an equivalent of untruncated sum-over-states expansion. The comparison of the performances between the first-order and second-order methods is shown for various molecules containing light to heavy elements, highlighting their relative strength and weakness. The formulations of QDPT and CP-CASSCF approaches as well as the derivation of the second-order Douglas–Kroll–Hess picture change of Zeeman operators are given in detail.

Using Monte Carlo Method to Study Magnetic Properties of Frozen Ferrofluid

Magnetic nanoparticles are single-domain particles of ferromagnetic or ferrite materials. Recently, magnetic nanoparticles have been applied more and more in technology, such as spintronics, magnetic recording, catalyst, and biomedicine. Therefore, experimental and theoretical studies on their magnetic properties are very important to provide essential information for individual applications. In addition, technical preparations have been developed fast, such as chemical synthesis, sputtering, or lithography. Depending on characters of assemblies, magnetic properties are different, such as twoor three-dimension, metallic or metallic oxide materials, order or disorder arrangement, surrounded by solids (granular solids) or liquids (ferrolfuids), and the magnetic or non-magnetic surrounding matrix. This leads to studying fundamental properties of magnetic nanoparticle assemblies becomes interesting. Among applicable potentials of magnetic nanoparticles, the biomedicine is a promising area, because the magnetic nanoparticles offer some great possibilities (Pankhurst et al., 2003). First, their size ranges from a few nanometers up to tens of nanometers. This means that their size can be smaller than of comparable to the size of biological entities, for example, a cell (10 – 100 μm), a virus (20 – 450 nm), a protein (5 – 50 nm) or a gene (2 nm wide and 10 – 100 nm long). Thus, they can penetrate easily into these entities. Second, these particles behave magnetic properties, so they can be controlled by an external magnetic field gradient. This opens application including the transport (drug delivery, cell separation) or immobilization (hyperthermia, contrast agent). Third, these particles can strongly resonate to a radio field. This makes them be easily excited by radio field leading, for example, heating in hyperthermia or magnetic resonance in contrast agent. A key quality to study magnetic properties of particle is magnetic anisotropy energy (MAE). However, it is very difficult to exactly observe the MAE of each particle in the assembly. We can just obtain the MAE distribution of the assembly. Usually, the MAE distribution f(EB) is deduced from the size distribution f(V) due to simplest expression of MAE, EB = KV, with K and V as anisotropy constant and volume, respectively, of each particle. However, this way does not describe the exact information of real systems. It is due to some reasons as follow. (i) The size distribution obtains from microscopy images may not coincide with the real sample. (ii) The magnetic anisotropy involves many complexities, such as surface, magneto-

Immobilising of anti-HPV18 and E. coli O157:H7 antibodies on magnetic silica-coated Fe3O4 for early diagnosis of cervical cancer and diarrhoea

This paper presents the synthesis and properties of magnetic silica-coated Fe3O4 nanoparticles. Uncoated Fe3O4 nanoparticles with an average diameter of 9-16 nm and saturation magnetisation of around 66 emu/g were first prepared by co-precipitation method. After being coated by SiO2 using the sol-gel method, the diameters of the coated particles ranged from 29 nm to 230 nm and their corresponding saturation magnetisation was reduced to 29 emu/g. The magnetic coated nanoparticles were attached to the monoclonal antibodies of HPV18 and E. coli O157:H7. The TG/DTA analyses indicated that antibodies were attached to the magnetic nanoparticles. The obtained results revealed that magnetic SiO2-coated Fe3O4 nanoparticles can be a promising candidate for the diagnosis of cervical cancer at an early stage with high accuracy.