Unitary Coupled Cluster Research Articles

Comparative studies using coupled-cluster and unitary coupled-cluster methods: nuclear quadrupole moment, hyperfine constants and transition properties of 27Al

The magnetic dipole and electric quadrupole hyperfine constants of Aluminium (27Al) atom are computed using the relativistic coupled cluster (CC) and unitary coupled cluster (UCC) methods. Effects of electron correlations are investigated using different levels of CC approximations and truncation schemes. The ionization potentials, excitation energies, transition probabilities, oscillator strengths and nuclear quadrupole moment are computed to assess the accuracy of these schemes. The nuclear quadrupole moment obtained from the present CC and UCC calculations in the singles and doubles approximations are 142.5 mbarn and 141.5 mbarn, respectively. The discrepancies between our calculated IPs and EEs and their measured values are better than 0.3%. The other one-electron properties reported here are also in excellent agreement with the measurements.

Algebraic propagator approaches and intermediate-state representations. I. The biorthogonal and unitary coupled-cluster methods.

As a general common concept, underlying diverse methods used to compute generalized electronic excitations in atoms and molecules, intermediate-state representations (ISR's), are considered and analyzed. Essentially, an ISR results by representing the excitation energy operator in terms of so-called correlated excited states (CES's) or states derived thereof. Three different ISR schemes are compared, namely the biorthogonal coupled-cluster (BCC) representation used in both the coupled-cluster linear response and equation-of-motion coupled-cluster methods, a unitary coupled-cluster (UCC) representation, and the excitation class orthogonalized (ECO) representation resulting from a Gram-Schmidt orthogonalization procedure for the CES. Moreover, the relationship between the BCC scheme and the symmetry-adapted-cluster--configuration-interaction method is discussed. The relevance of the ISR schemes, as opposed to the much simpler configuration-interaction (CI) expansions, arises from two basic properties referred to as separability and compactness. The former property is a sufficient condition for size-consistent results, while the latter allows one to use smaller explicit configuration spaces than in comparable CI treatments. We show that the ECO and UCC representations are both separable and compact, whereas a somewhat restricted compactness property applies in the BCC case. \textcopyright{} 1996 The American Physical Society.

Coupled-cluster, unitary coupled-cluster and MBPT(4) open-shell analytical gradient methods

We report open-shell analytical gradient methods and results for the high-level many-body methods, full fourth-order MBPT, coupled-cluster doubles, and the recently proposed unitary coupled-cluster method. Examples include vibrational frequencies and dipole moments of 3B 1 CH 2, 2B 1 NH 2, and 3B 1 SiH 2 in polarized basis sets.