Segmented contractions

There are two different ways of contracting a set of primitive GTOs to a set of contracted GTOs segmented and general contraction. Segmented contraction is the... 

In a segmented contraction each primitive as a rule is only used in one contracted function. In some cases it may be necessary to duplicate one or two PGTOs in two adjacent CGTOs. The contraction coefficients can be determined by a variational optimization, for example from an atomic HF calculation. 

All of the above basis sets are of the segmented contraction type. Modem contracted basis sets aimed at producing very accurate wave functions often employ a general contraction scheme. The ANO and cc basis sets below are of die general contraction type. 

Segmentation contractions occur as a result of the basic electrical rhythm (BER) of pacemaker cells in the small intestine. This form of muscular activity is slight or absent between meals. The motility of the small intestine may be enhanced during a meal by ... 

In contrast to segmentation contractions in the small intestine (9 to 12 per minute), haustral contractions occur much less frequently (up to 30 min between contractions). These very slow movements allow for the growth of bacteria in the large intestine. Normally, the bacterial flora in this region is harmless. In fact, some of the bacteria produce absorbable vitamins, especially vitamin K. 

One feature of the Pople basis sets is that they use a so-called segmented contraction. This implies that the primitives used for one basis function are not used for another of the same angular momentum (e.g., no common primitives between the 2s and 3s basis functions for phosphorus). Such a contraction scheme is typical of older basis sets. Other segmented split-valence basis sets include the MIDI and MAXI basis sets of Huzinaga and co-workers, which are named MIDI-1, MIDI-2, etc., MAXI-1, MAXI-2, etc. and vary in the number of primitives used for different kinds of functions. 

An alternative method to carrying out a segmented contraction is to use a so-called general contraction (Raffenetti 1973). In a general contraction, there is a single set of primitives that are used in all contracted basis functions, but they appear with different coefficients... 

For heavier elements primitive basis sets are available, but their contraction is a real art, at least if segmented contractions are used. The use of general contractions is far preferable and is much easier to carry out. 

Results obtained with a larger [5s 3p 2d If] segmented contracted basis axe given in Table 5.13 [89]. The effect of expanding the basis set is to increase the separation for all methods. Since the MRCI(0.025) calculation comprises about six million CSFs it is near the limit of what is currently feasible. The MRCI(O.Ol) calculation would involve over twenty million CSFs, which is impracticably large. Our best directly computed number is probably the MRCI+Q(0.025) result of a little over 5 kcal/mol. We can improve on this estimate by using the small basis results, however. 

Segmented contraction of basis sets. 157 Stochastical dynamics, 389 Unperturbed wave function, 123 model, 209 ... 

Codeine (t/ 3 h) activates opioid receptors on the smooth muscle of the bowel to reduce peristalsis and increase segmentation contractions. Tolerance may develop with prolonged use, as may dependence (rarely). It should be avoided in patients with... 

Mechanical Cutting vessel removes stretch with shortening (isometric to isotonic contraction) Reflex Segmental contraction of vessel Contraction by autopharmacodynamic agents SVPx, adrenalin, serotonin, histamine released locally from platelets, adrenochrome from sympathetic endings... 

Small Intestine Morphine diminishes biliary, pancreatic, and intestinal secretions and delays digestion of food in the small intestine. Resting tone is increased, and periodic spasms are observed. The amphtude of the nonpropulsive type of rhythmic, segmental contractions usually is enhanced, but propulsive contractions are decreased markedly. The upper part of the small intestine, particularly the duodenum, is affected more than the ileum. A period of relative atony may follow the hypertonicity. Water is absorbed more completely because of the delayed passage of bowel contents, and intestinal secretion is decreased this increases the viscosity of the bowel contents. 

Grivel, M.L. and Ruckebusch, Y. 1972. The propagation of segmental contractions alongthe smaUintestine. 

Cao XY, Dolg M (2002) Segmented contraction scheme for small-core lanthanide pseudo potential basis sets. J Mol Struct Theo Chem 581 139... 

F. E. Jorge, A. B. F. da SBva. A Segmented Contraction Methodology for Gaussian Basis Sets to be Used in Dirac-Fock Atomic and Molecular Calculations. Chem. Phys. Lett., 289 (1998) 469 72. 

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