A new internally contracted direct multiconfiguration–reference configuration interaction (MRCI) method is described which allows the use of much larger reference spaces than any previous MRCI method. The configurations with two electrons in the external orbital space are generated by applying pair excitation operators to the reference wave function as a whole, while the singly external and internal configurations are standard uncontracted spin eigenfunctions. A new efficient and simple method for the calculation of the coupling coefficients is used, which is well suited for vector machines, and allows the recalculation of all coupling coefficients each time they are needed. The vector H⋅c is computed partly in a nonorthogonal configuration basis. In order to test the accuracy of the internally contracted wave functions, benchmark calculations have been performed for F−, H2O, NH2, CH2, CH3, OH, NO, N2, and O2 at various geometries. The deviations of the energies obtained with internally contracted and uncontracted MRCI wave functions are mostly smaller than 1 mH and typically 3–5 times smaller than the deviations between the uncontracted MRCI and the full CI. Dipole moments, electric dipole polarizabilities, and electronic dipole transition moments calculated with uncontracted and contracted MRCI wave functions also are found to be in close agreement. The efficiency of the method is demonstrated in large scale calculations for the CN, NH3, CO2, and Cr2 molecules. In these calculations up to 3088 reference configurations and up to 154 orbitals were employed. The biggest calculation is equivalent to an uncontracted MRCI with more than 78 million configurations.

An efficient internally contracted multiconfiguration–reference configuration interaction method

A scheme that reduces the calculations of ground-state properties of systems of interacting particles exactly to the solution of single-particle Hartree-type equations has obvious advantages. It is not surprising, then, that the density functional formalism, which provides a way of doing this, has received much attention in the past two decades. The quality of the energy surfaces calculated using a simple local-density approximation for exchange and correlation exceeds by far the original expectations. In this work, the authors survey the formalism and some of its applications (in particular to atoms and small molecules) and discuss the reasons for the successes and failures of the local-density approximation and some of its modifications.

The density functional formalism, its applications and prospects

The Complete Active Space Self‐Consistent Field Method and its Applications in Electronic Structure Calculations

Techniques to support user interaction in a browser environment, “capture” certain user actions, and perform appropriate defined actions. In a first scheme, “setup” pages are used to detect user actions. Each setup page is associated with a respective script that is executed when the page is activated (e.g., via a Back or Forward button). The script performs the defined actions to obtain the proper behavior for dependent pages. The scripts for the setup pages keep track of the state of these pages, and further maintain a local history stack for the URLs of the dependent pages. In a second scheme, each dependent page includes a hidden history frame having a script that performs a set of defined actions for the page. The script further keeps track of whether or not the dependent page has been loaded previously.

Method and system for supporting user navigation in a browser environment

X-ray photoelectron spectroscopy: Towards reliable binding energy referencing

The interpretation of XPS spectra: Insights into materials properties

Size dependence of surface cluster models: CO adsorbed on Cu(100)

Analysis of ab initio cluster wave functions show that the shifts in the vibrational frequencies of CO/Cu and CN/Cu due to an applied electric field arise dominantly from a Stark effect. The CN bonding is largely ionic while CO has a dative covalent \ensuremath{\pi} bond. Consequently the Stark effect mechanisms are significantly different.

Field-induced vibrational frequency shifts of CO and CN chemisorbed on Cu(100).

The existence of a class of (very) high‐spin hydrocarbons is predicted on the basis of (a) new theorems for valence‐bond models, (b) classical structure theory, (c) standard theorems for molecular orbital theory, (d) several small‐system calculations with Hubbard and PPP Hamiltonians, and (e) a large‐system cluster expansion calculation applied to the polyallyl high‐spin candidates. Some properties, especially the magnetic ones, for these conjugated alternants are briefly discussed.

High‐spin hydrocarbons

The bonding in the 5,3Σ− and 3Δ states of FeCO, the 3,1Σ+, 3Δ, and 3 Π states of NiCO, and the 2Σ+ state of CuCO are analyzed using the constrained space orbital variation (CSOV) technique for both (CASSCF) and SCF wave functions. The bonding is discussed in terms of σ repulsion between the metal 4s and the CO 5σ, CO to metal σ donation when there is an empty or partly occupied dσ orbital and metal to CO 2π * backdonation. The bonding is compared for the different metals and between the different states.

Constance J. Nelin

Papers

The interpretation of XPS spectra: Insights into materials properties

Size dependence of surface cluster models: CO adsorbed on Cu(100)

Field-induced vibrational frequency shifts of CO and CN chemisorbed on Cu(100).

High‐spin hydrocarbons

The nature of the bonding in XCO for X=Fe, Ni, and Cu