High precision Delta(i) values at equilibrium determined by theoretical methods are imperatively needed as references for the development of new clumped-isotope thermometers (or tracers). In this study, quantum chemistry methods with corrections beyond the harmonic approximation are used to obtain the clumped-isotope signatures at equilibrium of several gas-phase molecules (i.e., CH4, NH3, H2O, H2S, and SO2). Here, we consider as many corrections to the traditional Bigeleisen-Mayer equation as possible to obtain accurate Delta(i) values at equilibrium and their temperature dependences. The corrections include anharmonic correction for zero-point energy, anharmonic correction for vibrational excited states, vibration-rotation coupling correction for zero-point energy, vibration-rotation coupling correction for vibrational excited states, quantum mechanical correction to rotation, and centrifugal distortion correction, which are important for theoretical understanding of clumped-isotope signals. Specifically, molecular constants are calculated via second-order perturbative analysis at the MP2/aug-cc-pVTZ level. The CCSD/6-311+G(3df,3pd) and CCSD/aug-cc-pVTZ levels are further employed to ensure the precision of harmonic frequencies of methane. For methane, a polynomial fit of Delta 13CH3D values over the temperature range of from 273.15 to 1000 K is obtained:

Delta 13CH3D = 0.00255 (1000/T)(4) = 0.11639 (1000/T)(3) +1.01364 (1000/T)(2) =0.43627 (1000/T)

Our results are slightly different from previous theoretical calculations, and may serve as new anchors for calibrating experimental observations. (C) 2015 Elsevier Ltd. All rights reserved.