Gian Luigi Bendazzoli

A vector and parallel full configuration interaction algorithm

A full configuration interaction (FCI) algorithm is presented and discussed. It is an integral driven formalism based on the explicit construction of tables which realize the correspondence between the FCI vector x and the vector Hx, H being the Hamiltonian matrix of the system. In this way no decomposition of the identity is needed, and in the simplest implementation only the two vectors x and Hx need to be kept on disk. The main test has been done on the cyclic polyene C18H18 in the Pariser–Parr–Pople approximation, where the size of the FCI vector can be reduced to about 73 million components. Running on a CRAY Y‐MP with 4 CPU and 32 MW of core memory, we obtained an elapsed CPU time per iteration of about 300 s and a total elapsed time of 1000 s, which correspond to about 4 and 14 s per million determinants, respectively. The parallel CPU speed‐up obtained by running with the 4 CPU is greater than 3, without any substantial increasing of the memory or disk requirements.

A full-configuration benchmark for the N2 molecule

Abstract A full-configuration interaction (FCI) calculation has been performed for the nitrogen molecule using an ANO [4s3p1d] basis set. The FCI space for such a system contains about 9.68×109 symmetry-adapted Slater determinants. The FCI results are compared with several approximate methods, both of single- and multi-reference type, in order to test their accuracy.

A new assignment of the uv spectrum of thiophene. Abinitio configuration interaction energies and the single crystal uv spectrum

The uv spectra of single crystals of thiophene have been recorded in the spectral region 38 000–50 000 cm−1 for temperatures down to 12 K. In the crystal spectrum the ? and ? absorption systems observed in the vapor phase have different polarization ratios and the ? system is not observed. Wavefunctions and energies have been computed for the lowest excited states by a relatively large orbital expansion and configuration mixing. The two intense electronic transitions are assigned to π–π* states of A1 and B2 symmetry and the intermediate absorption in the vapor spectrum is attributed to a Rydberg state.

Direct-List Algorithm for Configuration Interaction Calculations

We present a method for the direct generation of the lists of strings, suited for integral‐driven full‐CI (FCI) algorithms. This method generates the string lists each time they are used, and hence sensibly reduces the memory requirements, compared to our previous method that precalculates the lists. It was also extended to permit a truncation of the string space, according to the level of excitation. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 1329–1343, 1997

A theoretical study of linear beryllium chains: Full configuration interaction

We present a full configuration interaction study of Be(N) (N=2,3,4,5) linear chains. A comparative study of the basis-set effect on the reproduction of the energy profile has been reported. In particular, the 3s1p, 4s2p, 4s2p1d, 5s3p2d, and 5s3p2d1f bases were selected. For the smallest chains (i.e., Be(2) and Be(3)), smaller basis sets give dissociative energy profiles, so large basis set is demanded for the reproduction of equilibrium minima in the structures. For Be(4) and Be(5) linear chains, the energy profiles show a minimum also by using the smallest basis sets, but the largest ones give a much stronger stabilization energy. For all the structures, two spin states have been studied: the singlet and the triplet. It is shown that the energy separation of the two states, in the equilibrium region, is small and decays exponentially with respect to the number of atoms in the chain. Finally an interpolative technique allowing for the estimation of the long-chain parameters from shorter ones is presented.

A ONE BILLION DETERMINANT FULL CI BENCHMARK ON THE CRAY T3D

Abstract The implementation of an out-of-core version of a full CI algorithm on the Cray T3D is described. The introduction of heavy I/O activity, necessary to handle larger problems, required particular attention in order to maintain good performance. As an application, the FCI energy of the Be 2 molecule with a [9s2p1d] basis set (all electrons), whose FCI space has a dimension of more than one billion (10 9 ) of symmetry-adapted determinants in D 2h symmetry was computed. A single iteration on the Cray T3D at CINECA (64 processors) required about four hours, 30 minutes of which were spent in I/O operations. 27 iterations were performed and a precision of at least one μhartree in the energy was achieved. Due to the scalability of the code, substantially larger calculations could be performed provided that more processors and a larger amount of disk space were available.

A parallel Full-CI algorithm ✩

A Full Configuration Interaction (Full-CI) algorithm is described. It is an integral-driven approach, with on-the-fly computation of the string-excitation lists that realize the application of the Hamiltonian to the Full-CI vector. The algorithm has been implemented on vector and parallel architectures, both of shared and distributed-memory type. This gave us the possibility of performing large benchmark calculations, with a Full-CI space dimension up to almost ten billion of symmetry-adapted Slater determinants.

Beryllium Dimer: A Bond Based on Non-Dynamical Correlation

The bond nature in beryllium dimer has been theoretically investigated using high-level ab initio methods. A series of ANO basis sets of increasing quality, going from sp to spdf ghi contractions, has been employed, combined with HF, CAS-SCF, CISD, and MRCI calculations with several different active spaces. The quality of these calculations has been checked by comparing the results with valence Full-CI calculations, performed with the same basis sets. It is shown that two quasi-degenerated partly occupied orbitals play a crucial role to give a qualitatively correct description of the bond. Their nature is similar to that of the edge orbitals that give rise to the quasi-degenerated singlet-triplet states in longer beryllium chains.

Full configuration interaction study of the metal-insulator transition in model systems: LiN linear chains (N=2,4,6,8)

The precursor of the metal-insulator transition is studied at ab initio level in linear chains of equally spaced lithium atoms. In particular, full configuration interaction calculations (up to 1 x 10(9) determinants) are performed, in order to take into account the different nature of the wave function at different internuclear distances. Several indicators of the Metal-Insulator transition (minimum of the energy gap, maximum of the localization tensor or of the polarizability) are considered and discussed. It is shown that the different indicators give concordant results, showing a rapid change in the nature of the wave function at an internuclear distance of about 7 bohrs.

A full CI algorithm on the CRAY T3D. Application to the NH3 molecule

Abstract A full configuration interaction algorithm on the CRAY T3D massively parallel computer has been implemented. The algorithm shows good scaling behaviour as a function of the number of processors. As an application, we studied a problem that has attracted the attention of several authors in the past, i.e. the determination of the ground state energy of ammonia with a valence double-zeta plus polarization (vdzp) basis set.