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We optimized the coefficients for 11 multi-coefficient correlation methods (MCCMs) against 54 atomization energies for molecules composed of C, H, and O atoms and containing 343 bonds. The methods included two scaling-all-correlation (SAC) methods, a multi-coefficients SAC (MCSAC) method, three Utah methods, two Colorado methods, MC-QCISD, MCG3, and G3S. The mean unsigned errors are (on average) 36% lower than for calculations with general parameters. The mean unsigned error per bond for the most highly recommended methods is only 0.1−0.2 kcal/mol.

This paper presents a multi-coefficient correlation method based on quadratic configuration interaction with single and double excitations (MC-QCISD) and basis sets using segmented contraction and having the same exponential parameters in the s and p spaces. The results are comparable to a previous multi-coefficient correlation method based on coupled cluster theory with less efficient correlation-consistent basis sets, and they are better than a previous multi-coefficient correlation method based on Moller−Plesset fourth order perturbation theory with single, double, and quadruple excitations with correlation-consistent basis functions. The mean unsigned error per bond of the MC-QCISD method is 0.72 kcal/mol. The new method should be very efficient for computing geometries of open-shell transition states.

A new hybrid Hartree−Fock−density functional (HF-DF) model called the modified Perdew−Wang 1-parameter model for kinetics (MPW1K) is optimized against a database of 20 forward barrier heights, 20 r...

Geometry optimization has been carried out for three test molecules using six multilevel electronic structure methods, in particular Gaussian-2, Gaussian-3, multicoefficient G2, multicoefficient G3, and two multicoefficient correlation methods based on correlation-consistent basis sets. In the Gaussian-2 and Gaussian-3 methods, various levels are added and subtracted with unit coefficients, whereas the multicoefficient Gaussian-x methods involve noninteger parameters as coefficients. The multilevel optimizations drop the average error in the geometry (averaged over the 18 cases) by a factor of about two when compared to the single most expensive component of a given multilevel calculation, and in all 18 cases the accuracy of the atomization energy for the three test molecules improves; with an average improvement of 16.7 kcal/mol.

We calculate the rate constant for the reaction •H + CH3OH → H2 + •CH2OH both in the gas phase and in aqueous solution at 298 K. To accomplish this, we apply two different methods to estimate the electronic energies along the reaction path. First, we use specific reaction parameters (SRP) to mix the exchange and correlation energies in Becke's adiabatic connection theory (AC-SRP) to optimize the model for the specific bond-breaking, bond-making combination under consideration. Second, we obtain the potential energy using a linear combination of the Hartree−Fock method and AM1 with specific reaction parameters (HF||AM1-SRP); in this linear mixing method, eight NDDO parameters and the linear mixing parameter are simultaneously optimized by a genetic algorithm. To calculate the reaction rate constants in solution, the solute atomic charges are represented by class IV charges, the electric polarization of the solvent is determined from the electronic charge distribution of the solute self-consistently, and th...

We propose a multi-coefficient modification (MCG3) of the Gaussian-3 (G3) electronic structure method that is suitable for calculating continuous potential energy surfaces. We tested it for atomization energies and found that it improves the accuracy by 8% as compared to G3 and reduces the cost of single-point energy calculations by 50%. The method should be useful for calculating bond energies, potential energy surfaces, and thermochemical data of molecules.

Twelve general parameterizations of the SAC (scaling-all-correlation) method for semiempirical extrapolation of electronic structure calculations are presented. The methods are based on Moeller-Plesset perturbation theory and coupled-cluster theory with correlation-consistent basis sets, and the parameterizations are based on 49 equilibrium atomization energies. This paper also presents an optimized scale factor for estimating the total anharmonic zero-point vibrational energy of a molecule with a root-mean-square accuracy of 0.17 kcal/mol.

In this paper we propose a new method for calculating kinetic isotope effects without calculating a separate reaction path for each isotopically substituted species. The new method yields reasonable kinetic isotope effects from calculations using the same reaction path for all isotopic variations. The method is tested by carrying out variational transition state theory calculations, including multidimensional tunneling contributions, for the reactions OH+H2→H2O+H, CH3+H2→CH4+H, and H+H2→H2+H and nine deuterium-substituted isotopologs of these reactions. The results are very encouraging.

We present a new systematic set of algorithms for interpolated variational transition-state theory by mapping (IVTST-M). In this method, which is designed to allow efficient direct dynamics calculations, rate constants for chemical reactions are evaluated by variational transition-state theory with multidimensional tunneling approximations based on reaction-path data. The data (energies, energy gradients, and Hessians) are computed at a small number of points along a reaction path and fitted to splines under tension as functions of a mapped independent variable that is a nonlinear function of the reaction coordinate. The theory is illustrated and tested by several examples, and standard choices are employed for all parameters and functional forms to provide a realistic test of how the method might perform when applied as an automatic scheme without fine-tuning each reaction. For eight test cases, we obtain reasonable accuracy (as compared to calculations with the same potential surface with the reaction path followed as far as necessary for convergence) with Hessians at only six nonstationary points.

The Gaussian-2 method (G2) is modified by deleting the empirical high-level correction and instead using empirical coefficients to extrapolate to full configuration interaction and an infinite basis set. The resulting method, called multicoefficient Gaussian-2 (MCG2) is less expensive than G2 but a factor of 1.7 more accurate for molecules composed of H and first-row atoms.

Core correlation and scalar relativistic contributions to the atomization energy of 120 first- and second-row molecules have been determined using coupled cluster and averaged coupled-pair functional methods and the MTsmall core correlation basis set. These results are used to parametrize an improved version of a previously proposed bond order scheme for estimating contributions to atomization energies. The resulting model, which requires negligible computational effort, reproduces the computed core correlation contributions with 88%–94% average accuracy (depending on the type of molecule), and the scalar relativistic contribution with 82%–89% accuracy. This permits high-accuracy thermochemical calculations at greatly reduced computational cost.

The majority of the ab initio electronic structure calculations that are performed today include the correlation energy of only the valence electrons. This is sometimes called the frozen-core (FC) approximation. Even when correlated calculations are carried out without the FC approximation, one cannot make a realistic estimate of core correlation energy unless one adds extra basis functions in the core region, and standard basis sets 1 do not include these. The idea that only the valence electrons contribute to binding has a long and venerable history, dating to the early days of quantum chemistry, and it is essentially correct, rightfully constituting one of the key elements for understanding why the periodic table explains so much of chemistry. 2 However recent years have seen great progress in the calculation of molecular binding energies, 3 and in some cases errors due to neglecting the contribution of core correlation energy may exceed all other errors combined. In fact, over the past few years a number of researchers 4-17 have shown that the change in correlation energy of core electrons must be accounted for quantitatively to predict accurate molecular structures or energetics. Accounting for the correlation energy of core electrons by explicitly including them in correlated electronic structure calculations increases the computational cost by a very significant amount, due both to the need for larger one-electron basis sets and also to the larger number of manyelectron configurations that need be considered. This is prohibitively expensive for larger molecules. The purpose of the present paper is to point out that one can formulate a useful approximation to core correlation effects on binding energies by counting the heavy-atom bonds in a molecule.

The few controlled trials performed so far indicate that the addition of metformin and/or flutamide to a hypocaloric diet in obese women with polycystic ovary syndrome (PCOS) effectively influences different phenotypic aspects of the syndrome. All these studies are, however, characterized by a short to medium period of treatment.Our objective was to investigate the long-term effects of these therapies.We conducted a prospective, randomized, placebo-controlled trial at a medical center.Of 80 overweight-obese women with PCOS, 76 completed the study.Patients were placed on a hypocaloric diet for the first month and then on a hypocaloric diet plus placebo, metformin (850 mg, orally, twice a day), flutamide (250 mg, orally, twice a day), or metformin plus flutamide for the subsequent 12 months (20 subjects in each group).We assessed clinical features, computerized tomography measurement of fat distribution, androgens, lipids, and fasting and glucose-stimulated glucose and insulin levels at baseline and after 6 and 12 months of treatment.After 6 months, compared with placebo, flutamide further decreased visceral/sc fat mass (P = 0.044), androstenedione (P0.001), dehydroepiandrosterone sulfate (P0.001), and hirsutism score (P0.001), whereas metformin further increased frequency of menstruation (P = 0.039). After 12 months, flutamide maintained the effects observed after 6 months on visceral/sc fat mass (P = 0.033) and androstenedione (P0.001), whereas it produced an additional decrease in dehydroepiandrosterone sulfate (P = 0.020) and hirsutism score (P = 0.019); metformin further improved the menstrual pattern (P = 0.013). Moreover, after 12 months, flutamide improved more than placebo the menstrual pattern (P = 0.008), glucose-stimulated glucose levels (P = 0.041), insulin sensitivity (P0.001), and low-density lipoprotein cholesterol levels (P = 0.003), whereas metformin decreased glucose-stimulated insulin levels (P = 0.014). The combination of the two drugs maintained the specific effect of each of the compounds, without any additive or synergistic effect.These findings add relevance to the usefulness of metformin and flutamide in the treatment of dieting overweight-obese PCOS women and provide a rationale for targeting different therapeutic options according to the required outcomes in the long term.

The aim of this study was to evaluate the effect of selective and short-term sex hormone modifications on ghrelin levels in normal-weight eugonadal men undergoing hormonal contraceptive treatments. Seven men received an oral progestin [cyproterone acetate (CPA) or dienogest (DNG)] 10 mg/day for 3 weeks (CPA-DNG group), 7 CPA orally 5 mg/day in association with testosterone enanthate (TE) im 200 mg/week for 8 weeks (CPA-TE group), and 7 placebo (PLAC) for 8 weeks (PLAC group). Anthropometry and blood levels of LH, FSH, testosterone, estradiol, glucose, insulin and total ghrelin were evaluated. At baseline, no parameters differed among the three groups. After treatment, LH and FSH decreased in both CPA-DNG and CPA-TE groups, whereas they did not change in the PLAC group. Testosterone and estradiol decreased in the CPA-DNG group to the hypogonadal range, increased in the CPA-TE group to supraphysiological concentrations and, as expected, remained unchanged in the PLAC group. Total ghrelin levels increased in the CPA-DNG, decreased in the CPA-TE and did not change in the PLAC group. Ns modifications in the other parameters were observed in any group, demonstrating that the short-term changes of circulating sex hormones are able to modify ghrelin levels. These data, therefore, suggest that sex steroids are important regulators of ghrelin in normal-weight healthy men too.