Search results for "basis"
showing 10 items of 760 documents
Systematic conformational search analysis of the SRR and RRR epimers of 7-hydroxymatairesinol
2009
An extensive and systematic conformational search was performed on the two epimers of the natural lignan 7-hydroxymatairesinol (HMR), by means of a home-made Systematic Conformational Search Analysis (SCSA) code, designed to select more and more stable conformers through sequential geometry optimization of trial structures at increasing levels of calculation theory. In the present case, the starting molecular structures were selected by the semi-empirical AM1 method and filtered – i.e. decreased in number by choosing the more stable species – on the basis of their energy calculated by the HF method and the 6-31G(d) basis set. The geometries obtained were further refined by performing densit…
Exploring two-state reaction pathways in the photodimerization of cyclohexadiene.
2006
The ground- (S0) and lowest triplet-state (T1) pathways associated with dimerization of cyclohexadiene to give [2+2] and [4+2] cycloadducts have been theoretically studied at the UBLYP and UB3LYP levels of theory with the 6-31G* basis set. The DFT energies were validated by CCSD(T) single-point energy calculations. These cycloaddition reactions follow stepwise mechanisms with formation of bis-allylic biradical (BB) intermediates. In the S0 ground state, the interaction between two cyclohexadiene molecules with formation of BB intermediate IN(S0) has a large activation enthalpy of 32.0 kcal mol(-1). On the other hand, C-C bond-formation in the lowest triplet state (T1) leading to BB intermed…
Missing Curious Fraction Problems
2018
In this paper we present a study of one of the best-known types of descriptive word fraction problems. These problems have disappeared from today’s textbooks but are hugely important for developing arithmetic thinking. The aim of this paper is to examine the historical solution methods for these problems and discuss the analytical readings suggested by the authors. On the basis of this analysis we have conducted a preliminary study of the performance of 35 Spanish students who are highly trained in mathematics. Our results show that these students have a preference for algebraic reasoning, are reluctant to use arithmetic methods, and have reading comprehension difficulties that are reflecte…
Calculation of excitation energies from the CC2 linear response theory using Cholesky decomposition
2014
A new implementation of the approximate coupled cluster singles and doubles CC2 linear response model is reported. It employs a Cholesky decomposition of the two-electron integrals that significantly reduces the computational cost and the storage requirements of the method compared to standard implementations. Our algorithm also exploits a partitioning form of the CC2 equations which reduces the dimension of the problem and avoids the storage of doubles amplitudes. We present calculation of excitation energies of benzene using a hierarchy of basis sets and compare the results with conventional CC2 calculations. The reduction of the scaling is evaluated as well as the effect of the Cholesky …
On the intrinsic complexity of learning
1995
A new view of learning is presented. The basis of this view is a natural notion of reduction. We prove completeness and relative difficulty results. An infinite hierarchy of intrinsically more and more difficult to learn concepts is presented. Our results indicate that the complexity notion captured by our new notion of reduction differs dramatically from the traditional studies of the complexity of the algorithms performing learning tasks.
How can hepatitis C be prevented in the long term?
2007
Abstract Significant advances have been made in preventing HIV infection among injectors but we still know little about preventing hepatitis C (HCV). Both prevalence and incidence of hepatitis C can remain high among IDUs even in the context of widespread implementation of harm reduction programmes. We need to develop new ways to fill the knowledge gap regarding HCV prevention. One way is to learn from the experts—those IDUs who, after long-term injection in social milieus of high hepatitis C prevalence, nonetheless remain uninfected. We describe a recently commenced program of research that focuses on understanding the strategies, behaviours, and environmental factors associated with "stay…
A New Method of Velocity Estimation Based on Variable Temporal Basis Using Incremental Encoder
1997
Abstract This paper deals with analysis and synthesis of algorithms for digital conditioning of signals generated by incremental encoders to estimate velocity of rotating devices for control purposes. Main objectives are to obtain high accuracy at low and high velocity and low tracking delays during accelerations. A digital conditioning method is described, Which uses a polynomial of order n whose coefficients are updated so as to fit the n+1 most recent velocity data acquired on a variable temporal basis. Digital sinlulations and experimental findings are shown with the ainl to validate the proposed estimation method and compare it with other methods.
AQM generalized nyquist stability in multiple bottleneck networks
2005
Abstract The influence of multiple bottlenecks on the stability of Active Queue Management (AQM) controllers, usually configured on a single bottleneck basis is discussed. We consider a network scenario where RED is configured at each router according to previously developed control theoretic techniques. These configuration rules assure stability in a single bottleneck scenario. We show that instability may arise when two links become congested. We justify this result through a multiple bottleneck model using the Generalized Nyquist stability criterion.
Computational determination of the dominant triplet population mechanism in photoexcited benzophenone
2014
In benzophenone, intersystem crossing occurs efficiently between the S-1(n pi(star)) state and the T-1 state of dominant n pi(star) character, leading to excited triplet states after photoexcitation. The transition mechanism between S-1(n pi(star)) and T-1 is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S-1 -> T-1 and indirect S-1 -> T-2(pi pi(star)) -> T-1 ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths a…
Performance of revised STO(1M)-3G basis set for prediction of 5-fluorocytosine chemical shifts.
2019
Nuclear shieldings and chemical shifts of 5-fluorocytosine (5FC) were predicted in the gas phase and DMSO solution modeled by polarizable continuum model using B3LYP density functional and revised STO(1M)-3G basis set. For comparison, eight arbitrary selected basis sets including STO-3G and medium-size Pople-type and larger dedicated Jensen-type ones were applied. The former basis sets were significantly smaller, but the calculated structural parameters, harmonic vibrational frequencies, were very accurate and close to those obtained with larger, polarization-consistent ones. The predicted 13 C and 1 H chemical shieldings of 5FC and cytosine, selected as parent molecule, were acceptable (ro…