Papers by Keyword: Density Functional Theory (DFT)

Abstract: The thermodynamic properties of (B1-xCx)(N1-xCx) (x=0, 0.25, 0.5) are calculated by the ultra-soft pseud-potentials within local density approximation in frame of density functional theory with virtual crystal approximation and using the quasi-harmonic Debye model. The normalized relative volume V/V0, bulk modulus, thermal expansion coefficient α and heat capacity were computed with different temperature and pressure. Meanwhile the lower limit of the thermal conductivity κmin is also examined in details.

Abstract: Density-functional theory has been used to calculate the energetically global-minimum geometries and electronic states of Ag_nH₂S (n=2, 4, 6) clusters. The lowest-energy structures of Ag₂, Ag₄, Ag₆, Ag₂H₂S, Ag₄H₂S and Ag₆H₂S clusters were obtained, respectively. The calculation results show that the lowest-energy structures of Ag₂, Ag₄ and Ag₆ clusters are planar geometries. The binding energies of Ag_n(n=2, 4, 6) clusters are gradually increasing in our calculations. Compare the infrared spectrum peaks of Ag₄ cluster with that of Ag₆ cluster, which show that the peaks shift to shortwave. After adsorption, we found that the peaks shift to shortwave by comparison.

Abstract: All electronic structures and infrared adsorption spectra of AgnH2S (n=3, 5) clusters have been performed by using density functional theory. We obtain the lowest-energy structures of Ag3, Ag5, Ag3H2S and Ag5H2S clusters. The calculation results show that the lowest-energy structures of Ag3 and Ag5 clusters are planar geometries. The lowest-energy structures of Ag3H2S and Ag5H2S can be obtained by adsorbs immediately H2S on Ag3 and Ag5 clusters. The peak of infrared spectrum is 120cm-1 for Ag3 cluster, which is smaller than that of Ag5 cluster (180 cm-1). The peak of infrared spectrum is 350cm-1 for Ag3H2S cluster, which is larger than that of Ag5H2S cluster (290 cm-1). The comparison illustrates that adsorption H2S molecule make the peak of infrared spectrum shifting to shortwave.

Abstract: A series of chiral donor-acceptor helicenes have been designed using the molecular engineering of organic nonlinear optical materials. The geometries of donor-acceptor helicenes 1, 2, 3, 4, 5 are optimized using density functional theory (DFT-B3LYP) method at the 6-31g (d, p) basis set level. Based on the obtained stable molecular configuration, we adopt the TDHT/PM3 method and time-dependent density-functional theory (TD-DFT) to calculate the nonlinear optical (NLO) properties and electronic spectra of these molecules. Results show that molecule 4 has the largest static hyperpolarizability β_μ of 40.4×10^-30esu, which is enhanced by its two charge transfer orientations. In molecule 2, there exists two competitive charge transfers that weaken the second-order nonlinear optical effect of D-π-A structures greatly. The structural type of molecule 3, 4 and 5 are expected to be more efficient to achieve large β values.

Abstract: Structure transitions induced by selective solvents in diblock copolymer films are investigated by density functional theory. By changing the selectivity and the packing fraction of selective solvents, the disorder-lamellar transition, the position switched lamellar phase transitions and the reentrant transitions are observed. Phase diagrams are constructed to clarify these effects. The results provide an effective way to control the transitions between the different microstructures.

Abstract: Six modules (Y1~Y6) containing Y-type s-triazine based derivatives were optimized using density functional theory at B3LYP/6-31+G* level. On the basis of the optimized structure, electronic absorption spectrum was calculated with TD-DFT(TDB3LYP/6-31+G*) and the second-order nonlinear optical properties (NLO) were calculated with finite field (FF) method and coded programs. The results indicated that these molecules had good nonlinear optical properties with 106 order of magnitude a.u. (10-27 esu) of 0 value. Introducing different electron groups to the end of the tri-branched chain of this derivative would cause different results. Introduction of strong electron donating group would make a relatively large increase of u thus improving their second-order nonlinear optical properties, and making this derivative a good non-linear optical material.

Abstract: Three novel dyes (D1, D2 and D3) containing triphenylamine (TPA) unit as core and bearing different benzimidazole units as secondary electron-donors are designed. The geometries, electronic structures, and electronic absorption spectra of these dyes are studied by DFT and TD-DFT. The optimized results indicate that these dyes are all non-coplanar, which can help to inhibit the close intermolecular π-π stacking aggregation effectively. The lowest unoccupied molecular orbital (LUMO) energy levels of the dyes are higher than the conduction band edge of the TiO2, which ensures a high efficiency of electron transfer from these dyes to TiO2 electrode. As the highest occupied molecular orbital (HOMO) energy levels of these dyes are lower than those of I-/I-3, these molecules that lose electrons could be restored by getting electrons from electrolyte. The absorption spectra of these dyes are simulated, and the calculated results indicate that D3 can absorb more photons than those of D1, D2 and TPAR in the region from 250 to 580 nm, which should have the best performance of photo-to-electric conversion efficiency.

Abstract: We make clusters of atoms of the size of less than 1 nanometer by using the density functional theory and from that we obtain the bond lengths corresponding to the minimum energy configuration. We are able to optimize large clusters of atoms and find the vibrational frequencies for each cluster. This calculation provides us with a method to identify the clusters present in an unknown sample of a glass by comparing the experimental Raman frequency with the calculated value. We start with the experimental values of the Raman frequencies of PSe (Phosphorous-Selenium) glass. We calculate the structural parameters of PSe, P4Se, P2Se2, P4Se5, PSe4, P4Se3 clusters of atoms and tabulate the vibrational frequencies. We compare the calculated values with those measured. In this way we find the clusters of atoms present in the glass. Some times, the same number of atoms can be rearranged in a different symmetry. Hence we learn the symmetries of molecules. We find that certain symmetries are broken due to self-organization in the glassy state.

Abstract: The electronic and reaction properties of natural sphalerite containing seven typical kinds of impurities is studied by the density-functional theory (DFT). Mn and Ga impurities result in the Mulliken charge of the Zn atom reduced and consequently might decrease the ionicity of sphalerite; while Fe, Cd, In, Ge and Tl impurities lead to the increase in the Mulliken charge of the Zn atom. The frontier orbital calculations show that Mn impurities might lead to the over-oxidization of sphalerite, which may cause the further oxidation of lead xanthate, and thus are unfavorable for the flotation of sphalerite.

Abstract: Our message is oxidation process must be minimized as possible. Many carbon-related defect structures are reported in SiC/SiO2 interface. In this paper, we investigated the effect of oxidation to the defect forming by density functional theory (DFT). In the result, we found carbon defect structure that completely different from in the present report. This defect structure has carbon-carbon single bond with no dangling bond. To see the forming process, compressive strain from inserted oxygen atoms induce the rearrangement of structure and cause C-C defect structure. We can know that this structure is formed with energy gain about 3.8eV. And this C-C defect induces trap state under the conduction bottoms.