QCISD


QEXHAUST heat duty for heat engine exhaust (kJ  [c.479]

The many approaches to the challenging timestep problem in biomolecular dynamics have achieved success with similar final schemes. However, the individual routes taken to produce these methods — via implicit integration, harmonic approximation, other separating frameworks, and/or force splitting into frequency classes — have been quite different. Each path has encountered different problems along the way which only increased our understanding of the numerical, computational, and accuracy issues involved. This contribution reported on our experiences in this quest. LN has its roots in LIN, which  [c.256]

The G3 method is still rather computationally intensive and so some efforts have been made to reduce the computational requirements whilst retaining an acceptable level of error. The G3(MP2) variant [Curtiss et al. 1999] replaces the MP4 calculations (which are particularly time-consuming), with comparable calculations at the MP2 level. This leaves the QCISD(T) stage as the most demanding step. The average absolute deviation of the energies calculated using the G3(MP2) method was 1.89 kcal/kmol on the entire 299 test systems, a significantly less accurate result than that of the full G3 method, but still noteworthy.  [c.137]

B) Filtration of hot solutions. The quickest method of removing traces of insoluble impurities from a hot solution is to  [c.11]

However, one can go too far in delineating science, as did Thomas Kuhn, who contended that all science reflects not the truth about nature but merely the scientists prevailing opinion, which is always subject to change. Science has, however, established many fundamental observations and facts of our physical world. For example, atoms exist in a variety corresponding to the elements, as do DNA, bacteria, stars and galaxies, gravity and electromagnetism, natural selection and evolution. Science is our quest for understanding of the physical world, and we should keep this in proper perspective while admitting to the limits of where our human understanding can reach.  [c.13]

What we now call chemistry slowly emerged over the centuries as mankind s use of varied substances and compounds and the quest for understanding of the material world evolved. The practical beginning of chemistry goes back to ancient Egypt, based on experience gained 111 metals, glass, pottery, tanning and dying substances, etc. On the other hand, speculations by the Greeks and peoples in the East laid the foundation of this quest for a better understanding into the nature of the material world.  [c.23]

He therefore wanted to synthesise TM 29 to check. Even with modem spectroscopic methods the quickest way to check the identity of a compound will often be to synthesise it by an unambiguous route and compare the n.m.r. and fingerprint i.r. spectra. How then would you make TM 29  [c.12]

Quadratic configuration interaction calculations (QCI) use an algorithm that is a combination of the Cl and CC algorithms. Thus, a QCISD calculation is an approximation to a CCSD calculation. These calculations are popular since they often give an optimal amount of correlation for high-accuracy calculations on organic molecules while using less CPU time than coupled cluster calculations. Most popular is the single- and double-excitation calculation, QCISD. Sometimes, triple excitations are included as well, QCISD(T). The T in parentheses indicates that the triple excitations are included perturbatively.  [c.26]

B) Filtration of hot solutions. The quickest method of removing traces of insoluble impurities from a hot solution is to  [c.11]

Doyle, J. L., and Bondurant, P. D. (1991) Development of an Automated, Laser-Based Gun Tube Inspection System, QUEST Technical Report No. 542, December.  [c.1067]

So far, we have talked about the internal motions of molecules which are exliibiting their natural behaviour, either isolated in the gas phase or surrounded by a bath in a condensed phase. These natural motions are inferred from carefidly designed spectroscopic experiments that are sufficiently mild that they simply probe what the molecule does when left to follow its own lights . However, there is also a great deal of effort toward using high-mtensity, carefiilly sculpted laser pulses which are anything but mild, in order to control the dynamics of molecules. In this quest, what role will be played by knowledge of their natural motions  [c.77]

Knowledge of internal molecular motions became a serious quest with Boyle and Newton, at the very dawn of modem natural science. Flowever, real progress only became possible with the advent of quantum theory in the 20th century. The study of internal molecular motion for most of the century was concerned primarily with molecules near their equilibrium configuration on the PES. This gave an enonnous amount of inunensely valuable infonuation, especially on the stmctural properties of molecules.  [c.80]

A Cl calculation is variational the energy obtained is guaranteed to be greater than the true energy. A drawback of Cl calculations other than those performed at the full Cl level is that tliey are not size consistent. Simply put, this means that the energy of a number N of noninteracting atoms or molecules is not equal to N times the energy of a single atom or molecule. Another consequence of size consistency is that, as the bond length in a diatomic molecule increases to infinity, so the energy of the system should become equal to the sum of tire energies of the respective atoms. To illustrate why this lack of size consistency arises, consider CID calculations on Be2 and on two beryllium atoms. The electronic configuration of Be is ls 2s and so if we label the two atoms A and B, then the wavefunction for each of the tw o separated atoms will include the configuration 1sa2pa1sb2pb (s1sa1sb2pa2pb), in which two electrons have been promoted in each beryllium atom from the 2s to the 2p orbitals. This configuration represents a quadruple excitation for the beryllium dimer, which has the electronic configuration 1sa1sb2sa2sb. This quadruply excited configuration is not included in the CID wavefunction for the dimer, which is restricted to double excitations. In fact, the energy of a Q calculation including only doubly excited states is expected to scale in proportion to VN, where N is the number of non-interacting species present, rather than N. The Quadratic Configuration Interaction method (QCISD) was introduced to try to deal with this it can be considered a size-consistent QSD theory [Pople et al. 1987]. The procedure involves the addition of higher excitation terms which are quadratic in their expansion coefficients. Higher still in theory is QCISD(T), in which an estimated contribution from the triple excitations can be incorporated, though with extra computational expense.  [c.133]

For a basis set with K basis functions, there are K(K — l)(fC — 2) K — 3) integrals of type (eb cd), but due to symmetry only one-eighth of these are rmique as shown. Similarly, there are 2K K - 1)(K - 2) of type (2) 4K(K - 1)(K - 2) of type (3) K K - 1) of type (4) 2K(K — 1) of type (5) 4K(K — 1) of type (6) and K of type 7. Thus, a basis set with 200 functions has a total of 202015050 unique two-electron integrals. For all but the smallest of basis sets most integrals are of type (1) which is why an ab initio problem is often considered to scale as K /8 (200 /8 = 200000000). Including electron correlation adds significantly to the computational cost for example, MP2 calculations scale as the fifth power of the number of basis functions. Electron correlation methods may also require significantly more memory and disk than the comparable SCF calculation the higher levels scale as the sixth power, and in QQSD(T), one part of the calculation is seventh order.  [c.139]

The purpose of this projeet is to gain familiarity with the strengths and limitations of the Gauss-Seidel iterative method (program QGSEID) of solving simultaneous equations.  [c.54]

If we look back on the history of human efforts to understand our world and the universe, these look like lofty goals that, I believe, mankind will never fully achieve. In earlier times, things were more simple. The great Greek thinkers and those who followed in their footsteps were able to combine the knowledge available of the physical world with their thoughts of the spiritual world and thus develop their overall philosophy. This changed with the expansion of scientific inquiry and quest for knowledge in the seventeenth century. By the twentieth century, few philosophers, except those who also had some background in the sciences, could claim sufficient knowledge of the physical world to even attempt serious consideration of its meaning. This opened the claim to center stage to some scientists, particularly physicists, suggesting that only science can attempt to give answers to such fundamental questions as the origin and meaning of the universe, life, our being as intelligent species and the understanding of the universal laws governing the physical and biological world. In reality, however, humankind with all its striving for such knowledge probably will never reach full understanding. For me this is readily acceptable. It seems only honest to admit our limitations because of which human knowledge can reach only a certain point. Our knowledge will continue to expand, but it hardly can be expected to give answers to many of the fundamental questions of mankind. Nuclear seientists developed insights in the ways in which the atoms of the elements were formed after the initial big bang, but chemists are coneerned with the assembly into moleeules (eompounds, materials) and their transformations. They can avoid the question of whether all this was planned and  [c.1]

I have spent my life in science pursuing the magic of chemistry. In attempting to give some perspectives and thoughts on science, it is first necessary to define what science really is. As with other frequently used (or misused) terms (such as God or democracy ) that have widely differing meanings to different people at different times and places, science does not seem to be readily and uniformly defined. Science, derived from the Latin scientia, originally meant general knowledge both of the physical and spiritual world. Through the ages, however, the meaning of science narrowed to the description and understanding (knowledge) of nature (i.e., the physical world). Science is thus a major intellectual activity of man, a search for knowledge of the physical world, the laws governing it, and its meaning. It also touches on fundamental, ageless questions as to our existence, origin, purpose, and intelligence and, through these, the limits of how far our understanding can reach. In many ways scientists intellectual efforts to express their thoughts and quest for general knowledge and understanding are similar to other intellectual efforts in areas such as the humanities and arts, although they are expressed in different ways.  [c.4]

One of the characteristics of intelligent life that developed on our planet is man s unending quest for knowledge. (I am using man as a synonym for humans without gender differentiation.) When our early ancestors gazed upon the sun and the stars, they were fascinated with these mysterious celestial bodies and their movement. Ever since, man has strived to understand the movement of heavenly bodies. But it was only such pioneers as Copernicus, Kepler, and Galileo who established the concepts of celestial mechanics, which eventually led to Newton s theory of gravitation. Physics thus emerged as a firm science in the seventeenth century.  [c.8]

These days we consider alchemy a strange and mystical mixture of magic and religion, at best an embryonic form of chemistry but more a pseudo-science. But as Jung pointed out, alchemy was not simply a futile quest to transform base metals into noble gold. It was an effort in a way to purify the ignoble and imperfect human soul and raise it to its highest and noblest state. It was thus in a way a religious quest —not necessarily just a scientific one. Matter and spirit were inseparable to medieval alchemists, and they strove to transform them through these procedures, which sometimes amounted to sacramental rites and religious rituals as much as scientific research.  [c.24]

It soon became clear that this project would be very different from any writing I had done before. I recognized that my goal was not only to give autobiographical recollections of my life and my career in chemistry but also to express some of my more general thoughts. These touch on varied topics, including the broader meaning of science in the quest for understanding and knowledge as well as their limitations. Science as a human endeavor means the search for knowledge about the physical world. Inevitably, however, this leads to such fundamental questions of how it all started and developed Was there a beginning Was our being planned by a higher intelligence We struggle with these and related questions while trying to balance what we know through science and what we must admit is beyond us. My thoughts are those of a scientist who always tried to maintain his early interest in the classics, history, philosophy, and the arts. In recent years I have particularly tried to fill in some of the gaps a life actively pursuing science inevitably imposes constraints on the time that one can spend reading and studying outside one s own field of specialization. Of course, I realize only too well my limitations and the lack of depth in my background in some of these areas. Therefore, I have tried not to overreach, and I will limit my thoughts to my own understanding and views, however imperfect they may be.  [c.286]

LED precursors QIGHT GENERATION - LIGHT-EMITTING DIODES] (Vol 15)  [c.431]


See pages that mention the term QCISD : [c.200]    [c.137]    [c.26]    [c.309]    [c.309]    [c.313]    [c.314]    [c.315]    [c.20]    [c.13]    [c.15]    [c.25]    [c.26]    [c.131]    [c.26]    [c.23]    [c.52]    [c.108]    [c.162]    [c.275]    [c.282]    [c.321]    [c.330]    [c.330]    [c.523]    [c.570]    [c.570]    [c.585]   
Molecular modelling Principles and applications (2001) -- [ c.113 , c.117 , c.119 ]

Computational chemistry using the PC (2003) -- [ c.312 ]

Advanced organic chemistry Ч.1 (2000) -- [ c.26 ]

Modelling molecular structures (2000) -- [ c.207 , c.208 ]