XiuJun Wang

Combined first-principles calculation and neural-network correction approach for heat of formation

Despite their success, the results of first-principles quantum mechanical calculations contain inherent numerical errors caused by various intrinsic approximations. We propose here a neural-network-based algorithm to greatly reduce these inherent errors. As a demonstration, this combined quantum mechanical calculation and neural-network correction approach is applied to the evaluation of standard heat of formation ΔfH⊖ for 180 small- to medium-sized organic molecules at 298 K. A dramatic reduction of numerical errors is clearly shown with systematic deviation being eliminated. For example, the root-mean-square deviation of the calculated ΔfH⊖ for the 180 molecules is reduced from 21.4 to 3.1 kcal mol−1 for B3LYP/6-311+G(d,p) and from 12.0 to 3.3 kcal mol−1 for B3LYP/6-311+G(3df,2p) before and after the neural-network correction.

Accurate prediction of heat of formation by combining Hartree-Fock'density functional theory calculation with linear regression correction approach

A linear regression correction approach has been developed successfully to account for the electron correlation energy missing in Hartree-Fock calculation and to reduce the calculation errors of density functional theory. The numbers of lone-pair electrons, bonding electrons and inner layer electrons in molecules, and the number of unpaired electrons in the composing atoms in their ground states are chosen to be the most important physical descriptors to determine the correlation energy unaccounted by Hartree-Fock method or to improve the results calculated by B3LYP density functional theory method. As a demonstration, this proposed linear regression correction approach has been applied to evaluate the standard heats of formation DeltaH(f) (Theta) of 180 small-sized to medium-sized organic molecules at 298.15 K. Upon correction, the mean absolute deviation for the 150 molecules in the training set decreases from 351.0 to 4.6 kcal/mol and 360.9 to 4.6 kcal/mol for HF/6-31G(d) and HF/6-311+G(d,p) methods, respectively. For B3LYP method, the mean absolute deviations are reduced from 9.2 and 18.2 kcal/mol to 2.7 and 2.4 kcal/mol for 6-31G(d) and 6-311+G(d,p) basis sets, respectively.

Improving the accuracy of density-functional theory calculation: The genetic algorithm and neural network approach

The combination of genetic algorithm and neural network approach (GANN) has been developed to improve the calculation accuracy of density functional theory. As a demonstration, this combined quantum mechanical calculation and GANN correction approach has been applied to evaluate the optical absorption energies of 150 organic molecules. The neural network approach reduces the root-mean-square (rms) deviation of the calculated absorption energies of 150 organic molecules from 0.47 to 0.22 eV for the TDDFTB3LYP6-31G(d) calculation, and the newly developed GANN correction approach reduces the rms deviation to 0.16 eV.

Electronic structure and charge distribution of potassium iodide intercalated single-walled carbon nanotubes

Recently, potassium iodide was inserted into single-walled carbon nanotubes. We present here a first-principles density-functional theory calculation of the electronic and optical properties of a potassium iodide intercalated (10,10) nanotube. Band structure, density of states, and charge distribution of the intercalated nanotube are determined. Significant changes in the electronic structure of carbon nanotube are found upon the intercalation. In particular, the electron distribution on the tube becomes more diffusive, and one out of every four K4s electrons transfers to the tube wall, while the other three go to I5p orbitals.

Localized-density-matrix, segment-molecular-orbitals and poly(p-phenylenevinylene) aggregates

The segment-molecular-orbital representation is developed and incorporated into the recently developed linear-scaling localized-density-matrix method. The entire system is divided into many segments, and the molecular orbitals of all segments form the basis functions of the segment-molecular-orbital representation. Introduction of different cutoff lengths for different segment-molecular-orbitals leads to a drastic reduction of the computational cost. As a result, the modified localized-density-matrix method is employed to investigate the optical responses of large Poly(p-phenylenevinylene) aggregates. In particular, the interchain excitations are studied. The complete neglect of differential overlap in spectroscopy hamiltonian is employed in the calculation.

Localized-density-matrix method and its application to nanomaterials

The localized-density-matrix (LDM) method has been developed to calculate the excited state properties of very large systems containing thousands of atoms. It is particularly suitable for simulating the dynamic electronic processes in nanoscale materials, and has been applied to poly(p-phenylenevynelene) (PPV) aggregates and carbon nanotubes. Absorption spectra of PPVs and carbon nanotubes have been calculated and compared to the experiments.

Current transport mechanism in InGaP∕GaAsSb∕GaAs double-heterojunction bipolar transistors

We have developed InGaP∕GaAsSb∕GaAs double-heterojunction bipolar transistors (DHBTs) with low turn-on voltage and high current gain by using a narrow energy bandgap GaAsSb layer as the base and an InGaP layer as the emitter. The current transport mechanism is examined by measuring both of the terminal currents in forward and reverse mode. The results show that the dominant current transport mechanism in the InGaP∕GaAsSb∕GaAs DHBTs is the transport of carriers across the base layer. This finding suggests that the bandgap offset produced by incorporating Sb composition into GaAs mainly appears on the valence band and the conduction-band offset in InGaP∕GaAsSb heterojunction is very small.

Thermal stability of current gain in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors

The thermal stability of current gain in InGaP∕GaAsSb∕GaAs double-heterojunction bipolar transistors (DHBTs) is investigated. The experimental results show that the current gain in the InGaP∕GaAsSb∕GaAs DHBTs is nearly independent of the substrate temperature at collector current densities >10A∕cm2, indicating that the InGaP∕GaAsSb∕GaAs DHBTs have excellent thermal stability. This finding suggests that the InGaP∕GaAsSb∕GaAs DHBTs have larger emitter-base junction valence-band discontinuity than traditional GaAs-based HBTs.