Xiao-Guang Ma

First-principles study of structure and quantum transport properties of C20 fullerene

Using first-principles density-functional theory and nonequilibrium Greens function formalism for quantum transport calculation, we study the electronic and transport properties of C(20) fullerene molecule. Our results show that the equilibrium conductance of C(20) molecule is near 1G(0). It is found that the I-V curve displays a linear region centered about V = 0 and nonlinear behavior under higher bias voltages and an obvious negative differential resistance phenomenon in a certain bias voltage range. The mechanism for the negative differential resistance behavior of C(20) is suggested. The present findings could be helpful for the application of the C(20) molecule in the field of single molecular devices or nanometer electronics.

Synchronization transitions on complex thermo-sensitive neuron networks with time delays

We have numerically studied the firing synchronization transitions on random thermo-sensitive neuron networks in dependence on information transmission delay tau, network randomness p, and coupling strength g. It is found that as tau is increased the neurons can exhibit transitions from burst synchronization (BS) to clustering anti-phase synchronization (APS), and further to spike synchronization (SS). It is also found that, with increasing p or g, there are transitions from spatiotemporal chaos to BS, then to APS, and finally to SS. However, the APS state with p or g exists only for intermediate tau values within a narrow range. For tau values outside this range, the APS state does not appear and the firings change directly from spatiotemporal chaos to BS or SS. These results show that, as time delay can do, network topology and coupling strength can also cause complex synchronization transitions in the neurons. In particular, the novel phenomenon of APS state with p or g shows that, with the help of appropriate random connections or coupling strength, the neurons may exhibit the APS behavior at a certain time delay for which the APS does not appear originally. These findings imply that time delay, network randomness, and coupling strength may have subtle effects on the firing behaviors on neuronal networks, and thus could play important roles in the information processing in neural systems.

Non-Gaussian noise optimized spiking activity of Hodgkin–Huxley neurons on random complex networks

In this paper, we numerically study how the NGNs deviation q from Gaussian noise (q=1) affects the spike coherence and synchronization of 60 coupled Hodgkin-Huxley (HH) neurons driven by a periodic sinusoidal stimulus on random complex networks. It is found that the effect of the deviation depends on the network randomness p (the fraction of random shortcuts): for larger p (p>0.15), the spiking regularity keeps being improved with increasing q; while, for smaller p (p< 0.15), the spiking regularity can reach the best performance at an optimal intermediate q value, indicating the occurrence of deviation-optimized spike coherence. The synchronization becomes enhanced with decreasing q, and the enhancing extent for a random HH neuron network is stronger than for a regular one. These behaviors show that the spike coherence and synchronization of the present HH neurons on random networks can be more strongly enhanced by various other types of external noise than by Gaussian noise, whereby the neuron firings may behave more periodically in time and more synchronously in space. Our results provide the constructive roles of the NGN on the spiking activity of the present system of HH neuron networks.

Quasi-classical Trajectory Study of the Ne + H2 + → NeH+ + H Reaction Based on Global Potential Energy Surface

Using the multireference configuration interaction method with a Davidson correction and a large orbital basis set (aug-cc-pVQZ), we obtain an energy grid that includes 32 038 points for the construction of a new analytical potential energy surface (APES) for the Ne + H(2)(+) → NeH(+) + H reaction. The APES is represented as a many-body expansion containing 142 parameters, which are fitted from 31u2009000 ab initio energies using an adaptive nonlinear least-squares algorithm. The geometric characteristics of the reported APES and the one presented here are also compared. On the basis of the APES we obtained, reaction cross sections are computed by means of quasi-classical trajectory (QCT) calculations and compared with the experimental and theoretical data in the literature.

Theoretical characteristics of the bound states of M-X complexes (M=Cu, Ag, and Au, and X=He, Ne, and Ar)

The potential energy curves (PECs) of the bound states of M-X (M=Cu, Ag, and Au and X=He, Ne, and Ar) complexes have been calculated using the coupled cluster singles and doubles method with perturbative treatment of triple excitations. Large basis sets and bond functions, as well as the basis set superposition errors, are employed to obtain accurate PECs. The analytical potential energy functions (APEFs) are fitted using the PECs. The vibrational energy levels and the spectroscopic parameters for the complexes are determined using our APEFs and compared to the theoretical works available at present. We also find that the PECs are bound with similar van der Waals interactions, which implies that He, Ne, and Ar may be used for buffer-gas cooling; and Cu, Ag, and Au may be trapped with a similar method because Cu and Ag have been experimentally trapped with He buffer-gas cooling.

Transition and enhancement of synchronization by time delays in stochastic Hodgkin-Huxley neuron networks

In this paper, we study the effect of time delay on spiking synchronizations in Newman-Watts networks of stochastic Hodgkin-Huxley (HH) neurons. It is found that as @t is increased, the neurons exhibit transitions from spiking synchronization (SS) to clustering anti-phase synchronization (APS) and back to SS. Furthermore, the SS after the APS is enhanced with increasing time delay. For different patch sizes (channel noise strength), network randomness (fraction of random connections), and coupling strengths, the neurons exhibit similar synchronization transitions and the APS always occurs at around @t=4, representing that the time delay-induced APS behavior is robust to the channel noise, the number of random connections, and the coupling strength. A simple explanation for this phenomenon was given in terms of the relation of spiking time-period and time delay values. Since the information processing in the neurons are fulfilled by the spiking activity of the membrane potential and the spiking synchronization plays a crucial role in the spiking activity, our results may help us understand the effect of time delay as well as the interplay of channel noise and time delay on the spiking activity and hence the information processing in stochastic neuronal systems.

Influence of time delay and channel blocking on multiple coherence resonance in Hodgkin–Huxley neuron networks

Toxins such as tetraethylammonium (TEA) and tetrodotoxin (TTX) may reduce the number of working potassium and sodium ion channels by poisoning and making them blocked, respectively. In this paper, we study how channel blocking (CB) affects the time delay-induced multiple coherence resonance (MCR), i.e., a phenomenon that the spiking of neuronal networks intermittently reaches the most ordered state, in stochastic Hodgkin-Huxley neuron networks. It is found that potassium and sodium CB have distinct effects. For potassium CB, the MCR occurs more frequently as the CB develops, but for sodium CB the MCR is badly impaired and only the first coherence resonance (CR) holds and, consequently, the MCR evolves into a single CR as sodium CB develops. We found for sodium CB the spiking becomes disordered at larger delay lengths, which may be the reason for the destruction of the MCR. The underlying mechanism is briefly discussed in terms of distinct effects of potassium and sodium CB on the spiking activity. These results show that potassium CB can increase the frequency of MCR with time delay, but sodium CB may suppress and even destroy the delay-induced MCR. These findings may help to understand the joint effects of CB and time delay on the spiking coherence of neuronal networks.

An ab initio study of the ground and low-lying excited states of KBe with the effect of inner-shell electrons

The potential energy curves (PECs) of 1(2)Σ(+), 2(2)Σ(+), 1(2)Π, and 2(2)Π states of KBe are calculated using multireference configuration interaction method and large all-electron basis sets. Four sets of frozen core orbitals (FCOs) are considered to examine the effect of inner-shell correlation electrons on the molecular properties. The ro-vibrational energy levels are obtained by solving the Schrödinger equation of nuclear motion based on the ab initio PECs. The spectroscopic parameters are determined from the ro-vibrational levels with Dunham expansion. The PECs are fitted into analytical potential energy functions using the Morse long-range potential function. The dipole moment functions of the states for KBe calculated with different FCOs are presented. The transition dipole moments for KBe between 1(2)Σ(+) and 2(2)Σ(+) states, 1(2)Π and 1(2)Σ(+) states, and 2(2)Π and 1(2)Σ(+) states are also obtained.

Quasi-classical trajectory study of the N(2D) + H2(X1ΣG+) → NH(X3Σ-) + H(2S) reaction based on an analytical potential energy surface.

Using the multireference configuration interaction method with the Davidson correction and a large orbital basis set (aug-cc-pV5Z), we obtain an energy grid that includes 17,500 points for the construction of a new analytical potential energy surface (APES) for the N((2)D) + H(2)(X(1)Σ(g)(+)) → NH(X(3)Σ(-)) + H((2)S) reaction. The APES, which contains 145 parameters and is represented with a many-body expansion and a new switch function, is fitted from the ab initio energies using an adaptive nonlinear least-squares algorithm. The geometric characteristics of the reported APES in the literature and those of our APES are also compared. On the basis of the APES that we obtained, reaction cross sections are computed by means of quasi-classical trajectory calculations and compared with the experimental and theoretical values available in the literature.

Density Functional Theory Studies of Aun+(CH3OH)m (n = 3, 5, m = 1−5) Complexes

The structural, energetic, and electronic properties of gold ions adsorbed methanol, Au(3)(+)-(CH(3)OH)(m) (m = 1-3) and Au(5)(+)-(CH(3)OH)(m) (m = 1-5), have been investigated using density functional theory (DFT) within a generalized gradient approximation (GGA). The geometric parameters, vibrational frequencies, adsorption energies, and Mulliken charges are used to analyze the interactions between Au(3,5)(+) clusters and methanol molecules. The present calculations show that more than one methanol molecule can be adsorbed onto small clusters of gold ions and that this adsorption is different from that of single-molecule absorption. The red shift of the C-O stretching frequency decreases as the number of methanol molecules, m, increases or as gold cluster size increases. The positive charge on Au(3,5)(+) and coordination number of the adsorption sites on the gold cluster are the dominant factors responsible for the strength of the interactions. We obtained C-O stretching frequencies in Au(1,2)(+)-(CH(3)OH) complexes that are below 931 cm(-1), which provides theoretical evidence for the experimental observation by Dietrich et al. [J. Chem. Phys. 2000, 112, 752].