Potential Energy Surface And Methods For Computing Ro-vibrational Spectra Of CH₃F-H₂ And CO-H₂

Spectral detection is one of the most important ways to characterize the structure and properties of materials and has always been favored by experimental and theoretical scientists.Among them,the analysis of the vibrational spectra between weakly interacting systems has become a hot area of research.With the rapid development of science and technology today,the improvement of computer performance and the improvement of experimental equipment have made the weak interaction between research molecules a new height.The analysis of the vibration spectrum is both an opportunity and a challenge.First,you need to choose an exact ab initio method to get the energy of the system.Second,build an intermolecular potential energy surface model that includes the intra-molecular coordinates of the research system.At the end,it is also the most difficult part.It strictly expresses the kinetic energy and potential energy matrix elements of the Hamiltonian of the system and accurately solves the vibration and rotational energy levels of the system.The vibration spectrum of the theoretical prediction system guides the demonstration of experimental data and further explores the nature of matter.The research system of this topic is as follows:1 For linear-linear molecule such as CO-H₂ dimer.A five-dimensional ab initio potential energy surface?PES?for CO–H₂ that explicitly incorporates dependence on the stretch coordinate of the CO monomer has been calculated.Analytic four dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for?_CO=0 and 1 to the Morse/long-range?MLR?potential function form.These fits to 30 206 points have root-mean-square?RMS?deviations of 0.087 and0.082 cm^-1,and require only 196 parameters.The resulting vibrationally averaged PESs provide good representations of the experimental infrared data:for infrared transitions of para-H2–CO and ortho-H2–CO,the RMS discrepancies are only 0.007 and 0.023cm^-1,which are almost in the same accuracy as those values of 0.010 and 0.018 cm^-1obtained from full six-dimensional ab initio PESs of V12 Science 336,1147?2012?.The calculated infrared band origin shift associated with the fundamental of CO is-0.179cm^-1 for para-H₂–CO,which is the same value as that extrapolated experimental value,and slightly better than the value of-0.176 cm^-1 obtained from V₁₂ PESs.With these potentials,the path integral Monte Carlo algorithm and a first order perturbation theory estimate are used to simulate the CO vibrational band origin frequency shifts of CO in?paraH₂?_N–CO clusters for N=1–20.The predicted vibrational frequency shifts are in excellent agreement with available experimental observations.2 Based on the accurate potential energy surface of CO-H₂,we further studied CO-?pH₂?₂,CO-?oD₂?₂ and mixed CO-pH₂-He trimers.The microwave and infrared spectra of CO-?pH₂?₂,CO-?oD₂?₂ and mixed CO-pH₂-He trimers are predicted by performing exact bound state calculations on the global potential energy surfaces defined as the sum of accurately known two-body pH₂–CO or oD₂–CO?in Li et al.J Chem Phys139:164315,2013?,pH2–pH2 or oD2–oD2?in Patkowski et al.J Chem Phys 129:094304,2008?,and pH₂–He pair potentials.A total of four transitions have been reported to date,three in the infrared region,and one in the microwave region,which are in good agreement with our theoretical predictions.Based on selection rules,new transitions for J 3 have been predicted,and the corresponding transition intensities at different temperatures are also calculated.These predictions will serve as a guide for new experiments.The weak and tentatively assigned transitions are verified by our calculations.Three-body effects and the quality of the potential are discussed.A technique for displaying the three-dimensional pH₂ or oD₂ density in the body-fixed frame is used and shows that in the ground state,the two pH₂ or two oD₂ molecules are localized,while the He's are delocalized.3 For the linear and nonlinear H₂-CH₃F systems,a large amount of matrix spectral data has been obtained by experimentalists.However,many of the unresolved problems and phenomena require the help of theoretical scientists.Accurate analytical spectral data is based on potential energy surfaces.So far,no potential energy surface of the H₂-CH₃F system has been reported.On the one hand,it is hard to include the intra-molecular stretching normal mode of the CH₃F monomer.On the other hand,the anisotropy of the high-dimensional potential energy surface makes it difficult in fitting PES.There is no detailed matrix element for this complex.A first effective six-dimensional ab initio potential energy surface?PES?for CH3F–H2 which explicitly includes the intra-molecular Q₃ stretching normal mode of the CH₃F monomer is presented.The electronic structure computations have been carried out at the explicitly correlated coupled cluster level of theory CCSD?T?-F12a with an augmented correlation-consistent triple zeta basis set.Five-dimensional analytical intermolecular PESs for?_3?CH3F?=0 and 1 are then obtained by fitting the vibrationally averaged potentials to the Morse/Long-Range?MLR?potential function form.The MLR function form is applied to the nonlinear molecule-linear molecule case for the first time.These fits to 25 015 points have root-mean-square deviations of 0.74cm^-1 and 0.082cm^-1 for interaction energy less than 0.0cm^-1.Using the adiabatic hindered-rotor approximation,three-dimensional PESs for CH3F–para H2 are generated from the 5D PESs over all possible orientations of the hydrogen monomer.The infrared and microwave spectra for CH3F–paraH2 dimer are predicted for the first time.These analytic PESs can be used for modeling the dynamical behavior in CH₃F–?H₂?_N clusters,including the possible appearance of microscopic superfluidity.