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Electrical conductances of single molecules have been investigated in experimental and theoretical works to realize single molecular devices. However, full understanding of molecular conductance is not yet achieved because of junction-to-junction fluctuation of observed conductance and discrepancy between the observed and calculated conductances. The observed conductances of benzene-1,4-dithiolate (BDT) range from 10-3 to 10-2 G0, where G0 is the unit of conductance (2e2/h). On the other hand, conductances computed using the density functional theory (DFT) and nonequilibrium Green's function (NEGF) method range from 10-2 to 10-1 G0. Although there are clear differences in conditions between the experimental (room temperature and solution environment) and theoretical (0K and vacuum) works, the influence of solution and temperature on conductance has seldom been reported so far. In this study, we therefore address the solution and temperature effects on conductance using ab initio NEGF-DFT calculations. The computational system we consider here is Au(100)/BDT/Au(100) with 1g/cm3 water solution surrounding the BDT molecule. To generate reasonable configurations of water molecules, we performed two types of calculations: (1) relaxation of water molecules using the NEGF-DFT and (2) Car-Parrinello molecular dynamics (CPMD) simulations at room temperature. For each configuration of water molecules, we calculate the conductances of Au(100)/BDT+water/Au(100) using the NEGF-DFT, and obtain a time-averaged conductance in the second type of simulation. The calculated conductances of the BDT are 0.37, 0.31, and 0.32 G0 in the vacuum, water (relaxation), and water (CPMD), respectively, suggesting that the effects of water and temperature are not trivial. In particular, the dipole moments of water molecules affect the potential profile of the BDT molecule, leading to the clear difference in the tunneling barrier between the water solution and the vacuum cases. |