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1.
The potential use of small basis sets upon a low level of theory was studied on the calculations of electronic properties (dipole moment, static polarisability and static hyperpolarisabilities) of a series of 16 four-membered heterocycles with an exocyclic double bond at the position 3 (116). First, the calculations were performed within the Hartree–Fock (HF) approximation using 6-31G, 6-31G(d,p) and 6-31+G(d,p) as basis sets for the different conformational states of each molecule, and the results obtained were compared with the MP2/6-31+G(d,p) results reported. In the second place, in order to know the real potential of HF calculations, these were compared with those calculated using larger approaches such as MP2/6-311+G(d,p), MP2/aug-cc-pVDZ, CCS/6-311+G(d,p), CCS/aug-cc-pVDZ, CCS/aug-cc-pVTZ, CCSD/6-31+G(d) and CCSD/cc-pVDZ, taken into account only the planar and equilibrium geometries of each molecules. The HF approaches permit us to obtain a good qualitative representation of the dipole moment as a function of puckering angle in comparison with MP2, CCS and CCSD levels for all tested molecules. However, only HF/6-31+G(d,p) provides quantitative values of dipole moment for the heterocycles 1, 5 and 13 in comparison with MP2, CCS and CCSD levels. On the other hand, the polarisability and hyperpolarisabilities were quite sensitive to the quality of level of theory and basis sets. In particular, HF/6-31+G(d,p) predicted a representative approximation of alpha for the molecule 16 in comparison with larger methods as MP2/aug-cc-pVDZ, CCS/aug-cc-pVDZ and CCS/aug-cc-pVTZ, while a detailed analysis showed that HF can be used in the calculation of alpha for the molecules 3, 7, 11 and 15, but it requires the use of extended basis sets. Also, HF/6-31+G(d,p) predicted values of beta very similar to those calculated at the MP2 and CCSD levels used, for the planar and equilibrium geometry of the molecules 10 and 14. Furthermore, HF/6-31+G(d,p) described a representative dependence of this property with the puckering angle for the heterocycles 9, 10 and 14, compared with the MP2 curves.  相似文献   

2.
ABSTRACT

The method and basis set dependence of zero-point vibrational corrections (ZPVCs) to nuclear magnetic resonance shielding constants and anisotropies has been investigated using water as a test system. A systematic comparison has been made using the Hartree–Fock, second-order Møller–Plesset perturbation theory (MP2), coupled cluster singles and doubles (CCSD), coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)) and Kohn–Sham density functional theory with the B3LYP exchange-correlation functional methods in combination with the second-order vibrational perturbation theory (VPT2) approach for the vibrational corrections. As basis sets, the correlation consistent basis sets cc-pVXZ, aug-cc-pVXZ, cc-pCVXZ and aug-cc-pCVXZ with X = D, T, Q, 5, 6 and the polarisation consistent basis sets aug-pc-n and aug-pcS-n with n = 1, 2, 3, 4 were employed. Our results show that basis set convergence of the vibrational corrections is not monotonic and that very large basis sets are needed before a reasonable extrapolation to the basis set limit can be performed. Furthermore, our results suggest that coupled cluster methods and a decent basis set are required before the error of the electronic structure approach is lower than the inherent error of the VPT2 approximation.  相似文献   

3.
Interaction-induced electric dipole moment, polarisability and first hyperpolarisability are investigated in model hydrogen-bonded clusters built of hydrogen fluoride molecules organised in three linear chains parallel to each other. The properties are evaluated within the finite field approach, using the second order Møller–Plesset method, and the LPol-m (m = ds, dl) and the optical rotation prediction (ORP) basis sets. These bases and correlation method are selected after a systematic basis set and correlation method convergence study carried out on the smallest of the complexes and taking properties obtained with Dunning's bases and the coupled cluster singles and doubles (CCSD) and the CCSD including connected triple corrections (CCSD(T)) methods as reference. Results are analysed in terms of many-body and cooperative effects.  相似文献   

4.
A benchmark of explicitly correlated CCSD(T)-F12 methods for the dimers Ga2, As2, Br2 and diatomic molecules AsN, BrO, HBr, GaF, GaCl, GaBr, AsF, AsCl, BrF, and BrCl is presented. Equilibrium distances, harmonic vibrational frequencies, and dissociation energies are compared with extensive conventional CCSD(T) calculations using a variety of orbital basis sets and different ansätze for the explicitly correlated wavefunctions. Correlation of the 3d electrons has a strong effect, in particular on the equilibrium distances, and it is shown that this can be recovered very efficiently by the explicit correlation treatment. It is found that CCSD(T)-F12 calculations with new F12-specific cc-pVnZ-F12 basis sets give comparable accuracy to standard CCSD(T) calculations with very much larger aug-cc-pwCV(n+2)Z basis sets. The effects of higher order valence electron correlation (up to CCSDTQP) are also investigated in conventional calculations and are found to be significant in some cases.  相似文献   

5.
The utility of the recently developed extrapolation method to estimate the binding energies of weakly bound clusters at the basis set limit exploiting the similar basis set convergence behaviour of correlation energies of the monomer and cluster in correlated calculations (J. chem. Phys., 116, 5389 (2002)) was tested for small to medium (HF)n and (H2O)n (n = 2–5) clusters using various correlation consistent cc-pVXZ (X= D,T,Q,5) basis sets containing different numbers of diffuse functions and 6–31G type basis sets at the MP2 and CCSD(T) level for which accurate basis set limits are available for comparison. It is shown that the basis set limit binding energies estimated by this extrapolation method with modest size of basis sets (cc-pVDZ/cc-pVTZ or 6–3 1 G(d,p)/6-3 IG(2df,2pd)) are much closer to the exact basis set limits than the estimates by commonly used X ?3 extrapolation or counterpoise corrected binding energies, signifying the importance of this extrapolation method for the study of large weakly bound clusters. It is also shown that the inclusion of appropriate diffuse functions in the basis sets can significantly improve the accuracy of the estimated basis set limits by this extrapolation method. For (HF)n clusters the MP2 and CCSD(T) basis set limits estimated by this extrapolation method with aug-cc-pVDZ and aug-cc-pVTZ basis sets are 18.4 (18.5) and 18.9 (18.9) for the dimer, 61.8 (62) and 63.2 (63) for the trimer, 113.5 and 114.7 (116) for the tetramer, and 155.2 and 156.3 (158) for the pentamer, respectively, with the values in parentheses representing the apparent basis set limits, with the numbers in units of kJ mol?1. The corresponding results for (H2O)n clusters are 20.5 (20.5) and 20.6 (20.7) for (H2O)2, and 60.5 (61) and 60.1 (60) for (H2O)3, respectively.  相似文献   

6.
Nitai Sylvetsky 《Molecular physics》2019,117(9-12):1078-1087
ABSTRACT

The inner-shell correlation contributions to the total atomisation energies of the W4-17 computational thermochemistry benchmark have been determined at the CCSD(T) level near the basis set limit using several families of core correlation basis sets, such as aug-cc-pCVnZ (n?=?3–6), aug-cc-pwCVnZ (n?=?3–5) and nZaPa-CV (n?=?3–5). The three families of basis sets agree very well with each other (0.01?kcal/mol RMS) when extrapolating from the two largest available n: however, there are considerable differences in convergence behaviour for the smaller basis sets. nZaPa-CV is superior for the core-core term and awCVnZ for the core-valence term. While the aug-cc-pwCV(T+d)Z basis set of Yockel and Wilson is superior to aug-cc-pwCVTZ, further extension of this family proved unproductive. The best compromise between accuracy and computational cost, in the context of high-accuracy computational thermochemistry methods, is CCSD(T)/awCV{T,Q}Z, where the {T,Q} notation stands for extrapolation from the awCVTZ and awCVQZ basis set pair. For lower-cost calculations, we recommend a previously proposed combination of CCSD-F12b/cc-pCVTZ-F12 and CCSD(T)/pwCVTZ(no f). While in first-row molecules core-valence correlation on average accounts for over 90% of the inner-shell contribution, in second-row molecules core-core contributions may become important, particularly in systems like P4 and S4 with multiple adjacent second-row atoms.  相似文献   

7.
Geometry optimization and harmonic vibrational frequency calculations at the CASSCF, MP2 and CCSD(T) levels with basis sets up to 6-311G(2df) quality were carried out on the X1Σ+states of FCN and ClCN and the X2Π and A2Σ+ states of their cations. Adiabatic ionization energies were calculated up to the CCSD(T)/6-311G(3df)//CCSD(T)/6-311G(2d) level. Some B3LYP calculations were performed also for the ground states of the neutral molecules and the cations. Franck-Condon simulations were performed for the first two bands in the He I photoelectron spectra of FCN and ClCN by employing the ab initio computed geometries and frequencies. By comparing the observed and the simulated spectra obtained from different CN and CX (X = F or Cl) ionic bond lengths chosen on the basis of the ab initio computed values, the following structural parameters are obtained for the two lowest-lying states of FCN+ and ClCN+ (the method of deriving the uncertainties is described):  相似文献   

8.
Jiří Chmela 《Molecular physics》2018,116(12):1523-1538
Optimised auxiliary basis sets for lanthanide atoms (Ce to Lu) for four basis sets of the Karlsruhe error-balanced segmented contracted def2 - series (SVP, TZVP, TZVPP and QZVPP) are reported. These auxiliary basis sets enable the use of the resolution-of-the-identity (RI) approximation in post Hartree–Fock methods – as for example, second-order perturbation theory (MP2) and coupled cluster (CC) theory. The auxiliary basis sets are tested on an enlarged set of about a hundred molecules where the test criterion is the size of the RI error in MP2 calculations. Our tests also show that the same auxiliary basis sets can be used together with different effective core potentials. With these auxiliary basis set calculations of MP2 and CC quality can now be performed efficiently on medium-sized molecules containing lanthanides.  相似文献   

9.
利用耦合簇方法和Dunning等提出的系列相关一致基对PH2自由基的基态结构进行优化, 并使用优选出的cc-pV5Z基组对其进行频率计算. 结果表明,平衡核间距RP—H=0.14185 nm, 键角αHPH=91.8624°, 离解能De(HP—H)=3.483 eV, 对称伸缩振动频率ν1a1)=2399.9781 cm-1, 弯曲振动频率ν2a1)=1128.4213 cm-1,反对称伸缩振动频率ν3b2)=2407.8374 cm-1. 在此基础上采用多体项展式理论导出了PH2自由基的解析势能函数, 其等值势能图准确再现了PH2自由基分子的平衡结构特征和动力学特征. 关键词: 2自由基')" href="#">PH2自由基 多体项展式理论 解析势能函数  相似文献   

10.
《Molecular physics》2012,110(19-20):2503-2512
Using the coupled cluster singles and doubles method together with the recently developed LPol-n (n?=?ds, dl, fs, fl), the aug-pc-2, the SVPD, the TZVPD and Dunning's x-aug-cc-pVXZ basis sets, we calculate the interaction induced electric dipole moment and polarizability of the CO–Ne van der Waals complex. We consider the effect of extending the bases with different sets of mid-bond functions, and after a systematic basis set study carried out at a representative set of intermolecular geometries, we select the aug-cc-pVTZ, the aug-pc-2, the LPol-fs and the TZVPD bases with a set of 3s3p2d1f1g mid-bond functions placed in the middle of the van der Waals bond for the evaluation of the whole interaction induced property surfaces. After having determined the optimal parameters of appropriate analytical functions fitting the interaction induced properties, the resulting surfaces are used in semiclassical calculations of the dielectric and refractivity second virial coefficients of the system. All through this study the results obtained with Dunning's basis set hierarchy and reported in Phys. Chem. Chem. Phys., 11, 9871 (2009) are taken as reference.  相似文献   

11.
Highly correlated ab initio calculations with large basis sets are reported for difluorovinylidene, F(2)CC. Based on CCSD(T)/aug-cc-pVQZ results and taking core correlation effects properly into account, a reliable theoretical equilibrium geometry is derived: r(e)(CC) = 134.74(10) pm, r(e)(CF) = 131.00(10) pm, and angle(e)(FCC) = 123.23(10) degrees. The error bars are estimated from analogous comparative calculations on the equilibrium structures of the CF(2), C(2), and C(3) species. Correlated harmonic [CCSD(T)/aug-cc-pVQZ] and anharmonic [CCSD(T)/TZ2Pf] force fields provide theoretical values for the fundamental vibrational wavenumbers which are in excellent agreement with those measured previously in an argon matrix. Many spectroscopic constants of F(2)CC are predicted. In addition, the energy of F(2)CC relative to difluoroethyne (FCCF) and the barrier to isomerization from F(2)CC to FCCF have been reinvestigated by means of the present high-level ab initio calculations. Copyright 2001 Academic Press.  相似文献   

12.
A comparison is made of the accuracy with which the total electronic energy can be calculated by using either the finite basis set approach (the algebraic approximation) or finite difference methods in calculations employing the restricted Hartree-Fock model for the open-shell ground (X2Σ+) states of the Group Ha fluorides BeF, MgF, CaF and SrF. The convergence of the calculations carried out within the algebraic approximation is monitored by employing systematically constructed basis sets of increasing size. By using two different grids, the accuracy of the finite difference calculations has been estimated to be of the order of 10?2 μE h. The average difference between the finite basis set and finite difference total Hartree-Fock energies is 2.75 μEh. Dipole moments determined within the algebraic approximation are also compared with the corresponding finite difference expectation values.  相似文献   

13.
A new semi-empirical π-electron local coupled cluster theory has been developed to calculate static dipole polarisabilities and hyperpolarisabilities of extended π-conjugated systems. The key idea of the approach is the use of the ethylene molecular orbitals as the orbital basis set for π-conjugated compounds (the method is termed the Covalent Unbonded Molecules of Ethylene method, cue). Test calculations of some small model organic conjugated compounds demonstrate high accuracy of the version of the cue local coupled cluster theory developed in this work in comparison with the π-electron full configuration interaction (FCI) method. Calculations of different conjugated carbon-based oligomer chains (polyenes, polyynes, polyacenes, polybenzocyclobutadiene, etc.) demonstrate fast convergence (per π-electron) of the polarisability and hyperpolarisability values in the calculations when more classes of orbital excitations are included in the coupled cluster single and double (CCSD) excitation operator. The results show qualitatively correct dependence on the system size.  相似文献   

14.
使用Gaussian03程序包提供的CCSD(T)理论及Duning等的相关一致基cc-pVnZ和aug-cc-pVnZ (n=2, 3, 4, 5), 对SD(X2Π)自由基的平衡核间距、谐振频率及相互作用势进行了计算, 并拟合出了相应的光谱常数. 在CCSD(T)/aug-cc-pV5Z理论水平下, 光谱常数D0, De, R 关键词: 弹性碰撞 总截面 光谱常数 分子常数  相似文献   

15.
High-level ab initio electronic structure calculations up to the CCSD(T) theory level, including extrapolations to the complete basis set (CBS) limit, resulted in high precision energetics of the tautomeric equilibrium in 2-substituted acetaldehydes (XH2C-CHO). The CCSD(T)/CBS relative energies of the tautomers were estimated using CCSD(T)/aug-cc-pVTZ, MP3/aug-cc-pVQZ, and MP2/aug-cc-pV5Z calculations with MP2/aug-cc-pVTZ geometries. The relative enol (XHC?=?CHOH) stabilities (ΔE e,CCSD(T)/CBS) were found to be 5.98?±?0.17, ?1.67?±?0.82, 7.64?±?0.21, 8.39?±?0.31, 2.82?±?0.52, 10.27?±?0.39, 9.12?±?0.18, 5.47?±?0.53, 7.50?±?0.43, 10.12?±?0.51, 8.49?±?0.33, and 6.19?±?0.18?kcal?mol?1 for X?=?BeH, BH2, CH3, Cl, CN, F, H, NC, NH2, OCH3, OH, and SH, respectively. Inconsistencies between the results of complex/composite energy computations methods Gn/CBS (G2, G3, CBS-4M, and CBS-QB3) and high-level ab initio methods (CCSD(T)/CBS and MP2/CBS) were found. DFT/aug-cc-pVTZ results with B3LYP, PBE0 (PBE1PBE), TPSS, and BMK density functionals were close to the CCSD(T)/CBS levels (MAD?=?1.04?kcal?mol?1).  相似文献   

16.
Igor Novak 《Molecular physics》2017,115(8):925-930
The effects of ‘through-conjugation’ (TC) vs. ‘cross-conjugation’ (CC) have been studied in a series of [n]dendralene analogues with the aid of high-level molecular orbital (MO) calculations using composite G3MP2 method. The energy differences between TC and the corresponding CC alkenes have been determined and found to be small and comparable to the conformational barrier for rotation around the C–C single bond. The destabilisation in CC vs. TC alkenes per C = C unit does not show different trends within the even and odd series of [n]dendralenes. The observed differences in UV–vis and nuclear magnetic resonance (NMR) spectra and chemical reactivities between even and odd [n]dendralenes are thus solely due to conformational preferences which of course affect C = C bond conjugation (π-delocalisation). We also propose new partially hydrogenated graphene materials (‘dendraphenes’) which were introduced in the isodesmic reactions designed to study CC.  相似文献   

17.
ABSTRACT

We present new, highly accurate, measurements of the fundamental band of the roto-vibrational Raman spectrum of molecular nitrogen and oxygen. Experimentally obtained spectra were used as a basis to derive information about the anisotropy of the polarisability tensor, including its dependence on rotational state due to molecular non-rigidity. Third-order perturbative calculations were used to determine corrections to the Raman scattering cross sections as a function of the rotational quantum numbers. Our results for the ratio of polarisability anisotropy at equilibrium divided by its first derivative for nitrogen are in agreement with values reported previously and are significantly different from historical data obtained for oxygen.  相似文献   

18.
《Molecular physics》2012,110(19-20):2557-2567
It is well known that the convergence of correlation energies in atomic and molecular calculations is relatively slow and that calculations aimed at high accuracy must explicitly make corrections for this. In this work we consider 1e ? basis set extrapolation as a means of obtaining high accuracy. The correlation consistent basis sets of Dunning et al. have provided a convenient platform for extrapolation, with the independent variable being X?=?D,?T,?Q,?5,?…?. There has been much debate in the literature about the functional form to use for the extrapolation, with contention between the ‘theoretically justified’ (X?+?1)?3 form and empirical forms based on exponentials or variable powers. We will dissect the theoretical justification of the (X?+?1)?3 form by considering MP2 calculations on He and Ne as a function of the maximum angular momentum (?) in the basis using basis sets having converged radial extent. Calculations with ? up to 14 were carried out for Ne. It is shown that while the asymptotic form of (??+?1)?3 is clearly reached, higher order terms are very important in the range of ? normally used in molecular calculations. We also use similar analysis techniques for an open shell atom and a small molecule. The functional form for the dependence of molecular properties with ? is complex and it is safer to extrapolate fitting parameters than energies.  相似文献   

19.
Coupled Hartree-Fock calculations on the susceptibilities X and magnetic shielding constants σ are reported for the title molecules. Very large gaussian basis sets have been used in order to come sufficiently close to the Hartree-Fock limit. The basis set dependence of X and σ is discussed. In addition the applicability of two methods (maximum of X [20] and closure relations [21, 22]) for the prediction of the best gauge origins is investigated.

For C2H2, C2H4 aand C2H6 accurate theoretical results for the complete tensors of X and σ are given for the first time. For ethylene antishielding of σ yy C is observed and illustrated by means of the induced electric currents.  相似文献   

20.
He-HI复合物势能面及微分散射截面的理论研究   总被引:5,自引:1,他引:5       下载免费PDF全文
采用超分子单双迭代(包括非迭代三重激发)耦合簇理论CCSD(T)方法和由键函数3s3p2d1f组成的大基组, 计算得到了基态He-HI复合物相互作用的全程势能面. 该势能面上存在2个势阱, 分别对应于线性He-I-H和He-H-I构型, 势阱深度分别为4.473和2.996meV, He原子到HI分子质心的距离R分别为0.363和0.442nm. 使用Barker, Fisher和Watts提出的BFW势函数拟合计算得到的相互作用能数据, 获得了He原子与HI分子相互作用势的解析表达式. 在 关键词: He-HI复合物 势能面 微分散射截面  相似文献   

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