An accurate description of noncovalent interaction energies is one of the most challenging tasks in computational chemistry. To date, nonempirical CCSD(T)/CBS has been used as a benchmark reference. However, its practical use is limited due to the rapid growth of its computational cost with the system complexity. Here, we show that the fixed-node diffusion Monte Carlo (FN-DMC) method with a more favorable scaling is capable of reaching the CCSD(T)/CBS within subchemical accuracy (<0.1 kcal/mol) on a testing set of six small noncovalent complexes including the water dimer. In benzene/water, benzene/methane, and the T-shape benzene dimer, FN-DMC provides interaction energies that agree within 0.25 kcal/mol with the best available CCSD(T)/CBS estimates. The demonstrated predictive power of FN-DMC therefore provides new opportunities for studies of the vast and important class of medium/large noncovalent complexes.

Reliable theoretical predictions of noncovalent interaction energies, which are important e.g. in drug-design and hydrogenstorage applications, belong to longstanding challenges of contemporary quantum chemistry. In this respect, the fixed-node diffusion Monte Carlo (FN-DMC) is a promising alternative to the commonly used "gold standard" coupled-cluster CCSD(T)/CBS method for its benchmark accuracy and favourable scaling, in contrast to other correlated wave function approaches. This work is focused on the analysis of protocols and possible tradeoffs for FN-DMC estimations of noncovalent interaction energies and proposes an efficient yet accurate computational protocol using simplified explicit correlation terms with a favorable O(N 3 ) scaling. It achieves an excellent agreement (mean unsigned error ∼0.2 kcal/mol) with respect to the CCSD(T)/CBS data on a number of complexes, including benzene/hydrogen, T-shape benzene dimer, stacked adenine-thymine and a set of small noncovalent complexes A24. The high accuracy and reduced computational costs predestinate the reported protocol for practical interaction energy calculations of large noncovalent complexes, where the CCSD(T)/CBS is prohibitively expensive.

Large-scale quantum Monte Carlo (QMC) calculations of ground and excited singlet states of both conformers of azobenzene are presented. Remarkable accuracy is achieved by combining medium accuracy quantum chemistry methods with QMC. The results not only reproduce measured values with chemical accuracy but the accuracy is sufficient to identify part of experimental results which appear to be biased. Novel analysis of nodal surface structure yields new insights and control over their convergence, providing boost to the chemical accuracy electronic structure methods of large molecular systems.

Quantum Monte Carlo (QMC) is applied to obtain the fundamental (quasiparticle) electronic band gap, ∆ f , of a semiconducting two-dimensional (2D) phosphorene whose optical and electronic properties fill the void between graphene and 2D transition metal dichalcogenides. Similarly to other 2D materials, the electronic structure of phosphorene is strongly influenced by reduced screening, making it challenging to obtain reliable predictions by single-particle density functional methods. Advanced GW techniques, which include many-body effects as perturbative corrections, are hardly consistent with each other, predicting the band gap of phosphorene with a spread of almost 1 eV, from 1.6 to 2.4 eV. Our QMC results, from infinite periodic superlattices as well as from finite clusters, predict ∆ f to be about 2.4 eV, indicating that available GW results are systematically underestimating the gap. Using the recently uncovered universal scaling between the exciton binding energy and ∆ f , we predict the optical gap of 1.75 eV that can be directly related to measurements even on encapsulated samples due to its robustness against dielectric environment. The QMC gaps are indeed consistent with recent experiments based on optical absorption and photoluminescence excitation spectroscopy. We also predict the cohesion of phosphorene to be only slightly smaller than that of the bulk crystal. Our investigations not only benchmark GW methods and experiments, but also open the field of 2D electronic structure to computationally intensive but highly predictive QMC methods which include many-body effects such as electronic correlations and van der Waals interactions explicitly.

Singlet and triplet excited states of trans-azobenzene have been measured in the gas phase by electron energy loss spectroscopy (EELS). In order to interpret the strongly overlapping singlet and triplet bands in the spectra a set of large-scale correlated quantum Monte-Carlo (QMC) simulations was performed. The EELS/QMC combination of methods yields an excellent agreement between theory and experiment and for the two low-lying excited singlet and two low-lying triplet states permitted their unambiguous assignment. In addition, EELS revealed two overlapping electronic states in the band commonly assigned as S(2), the lower one with a pronounced vibrational structure, the upper one structureless. Finally, the agreement between theory and experiment was shown to further increase by taking computationally into account the finite temperature effects.

We present a construction of a matrix product state (MPS) that approximates the largesteigenvalue eigenvector of a transfer matrix T , for the purpose of rapidly performing the infinite system density matrix renormalization group (DMRG) method applied to two-dimensional classical lattice models. We use the fact that the largest-eigenvalue eigenvector of T can be approximated by a state vector created from the upper or lower half of a finite size cluster. Decomposition of the obtained state vector into the MPS gives a way of extending the MPS, at the system size increment process in the infinite system DMRG algorithm. As a result, we successfully give the physical interpretation of the product wave function renormalization group (PWFRG) method, and obtain its appropriate initial condition.

The axial next-nearest-neighbor Ising model is studied in two dimensions at finite temperature using the density matrix renormalization group. The model exhibits phase transition of the second-order between the antiphase in low temperature and the modulated phase in high temperature. Observing the domain wall free energy, we confirm that the modulation period in high-temperature side is well explained by the free-fermion picture.

The application of silicon nanoparticles (Si NPs) is very promising in various emerging technologies and for fundamental quantum studies of semiconductor nanocrystals. Heavily boron and phosphorus codoped fluorescent Si NPs can be fabricated with diameters of a few nanometers. However, very little is understood about the structure and origin of the fluorescence of these NPs. In this work, we perform a systematic time-dependent density functional study of hundreds of codoped Si NPs representing millions of configurations. We identify the most stable dopant configurations and a correlation between these configurations and their optical gaps. We find that particular dopant configurations result in emission in the second biological window, which makes these nanoparticles viable for deep-tissue bioimaging applications.