A linear response theory for excited state energies for systems with a strongly correlated ground state

1-8In this paper we apply our recently developed state-specific multi-reference coupled cluster based linear response method to compute excited state energies for systems whose ground state has a pronounced multi-reference character. The parent state-specific theory is built on complete active space reference functions, and is designed to treat quasi-degeneracy of varying degrees while bypassing the intruder problem. The response theory based on such a function offers thus a very convenient tool for generating potential energy surfaces (PES) for excited states where a traditional response formulation based on a single reference theory will fail due to the quas-idegeneracy in some regions of the PES and the effective hamiltonian based response methods would be beset by intruders. A succinct summary of the response formalism is presented to both underline the basic issues and the mode of approach. Illustrative numerical results are presented for the PES of the excited states as a function of the angle opening for the paradigm system H4 in trapezoidal geometry (H4), whose ground state requires a two-configuration description at the rectangular geomet ry, and has intruders at an intermediate trapezoidal geometry. A comparison with the full CI results in the same bas is indicates the efficacy of the method

A state-specific approach to multireference coupled electron-pair approximation like methods: development and applications

The traditional multireference (MR) coupled-cluster (CC) methods based on the effective Hamiltonian are often beset by the problem of intruder states, and are not suitable for studying potential energy surface (PES) involving real or avoided curve crossing. State-specific MR-based approaches obviate this limitation. The state-specific MRCC (SS-MRCC) method developed some years ago [Mahapatra et al., J. Chem. Phys. 110, 6171 (1999)] can handle quasidegeneracy of varying degrees over a wide range of PES, including regions of real or avoided curve-crossing. Motivated by its success, we have suggested and explored in this paper a suite of physically motivated coupled electron-pair approximations (SS-MRCEPA) like methods, which are designed to capture the essential strength of the parent SS-MRCC method without significant sacrificing its accuracy. These SS-MRCEPA theories, like their CC counterparts, are based on complete active space, treat all the reference functions on the same footing and provide a description of potentially uniform precision of PES of states with varying MR character. The combining coefficients of the reference functions are self-consistently determined along with the cluster amplitudes themselves. The newly developed SS-MRCEPA methods are size-extensive, and are also size-consistent with localized orbitals. Among the various versions, there are two which are invariant with respect to the restricted rotations among doubly occupied and active orbitals separately. Similarity of performance of this latter and the noninvariant versions at the crossing points of the degenerate orbitals imply that the all the methods presented are rather robust with respect to the rotations among degenerate orbitals. Illustrative numerical applications are presented for PES of the ground state of a number of difficult test cases such as the model H4, H8 problems, the insertion of Be into H2, and Li2, where intruders exist and for a state of a molecule such as CH2, with pronounced MR character. Results obtained with SS-MRCEPA methods are found to be comparable in accuracy to the parent SS-MRCC and FCI/large scale CI results throughout the PES, which indicates the efficacy of our SS-MRCEPA methods over a wide range of geometries, despite their neglect of a host of complicated nonlinear terms, even when the traditional MR-based methods based on effective Hamiltonians fail due to intruders

State-specific multi-reference coupled electron-pair approximation like methods: formulation and molecular applications

We present two variants of state-specific multi-reference coupled electron-pair type approximants (SS-MRCEPA) of our recently formulated state-specific multi-reference coupled-cluster (SS-MRCC) theory. Just like the parent SS-MRCC theory, these are formulated with a complete active space, and are rigorously size-extensive and size-consistent. They also bypass the intruder problem very efficiently. The efficacy of the methods is illustrated with the computation of the ground state potential energy surface of the trapezoidal H4 model, where the ground state requires a two-determinantal model space and the effective hamiltonian methods face intruders

Theoretical aids in screening candidates for atomic clocks: Illustration for Yb II

Highly accurate electronic-structure calculations for metastable electronic excited states are needed to supplement scant experimental data in screening possible systems for new higher-precision atomic clocks. We test the suitability of relativistic coupled-cluster methods for the low-lying 2Fo excited states of the Yb II ion by computing the ionization potentials (IP) for the 2S1/2 and 2Fo states of Yb I and the $[{\rm Xe}]4f^{14}6s\,(^{2}S_{1/2}) \longrightarrow [{\rm Xe}]4f^{13}6s^2\,(^{2}F^o_{j})$, j = 5/2, 7/2, electric octupole transition amplitudes. The calculations establish a minimum lifetime of six years and of 10− 1 s for the 2Fo7/2 and 2Fo5/2 states, respectively. In addition, computations for the lifetimes (τ) of its [Xe]4f146p(2Po) states are compared with high-precision experimental data as tests of the accuracy of our predictions. To our knowledge, this is the first relativistic ab initio estimate of the lifetime and ionization potential for the 2Fo states of Yb II, and the results demonstrate the suitability of these methods to aid in screening other candidates for atomic clocks

A study on thyroid profile in chronic liver disease patients admitted in a rural tertiary care hospital of West Bengal, India

Introduction: Abnormal thyroid function tests occur in various non-thyroidal illnesses, like chronic liver disease(CLD), where no pre-existinghypothalamic-pituitary and thyroid gland dysfunction is present. Available studies reported decreased T3 and fT3 in most cases. But a limi tedstudy reported the association of fT4, rT3 and TSH level with severity of CLD. Aim is to measure thyroid functions in patients with CLD and toassess the severity of liver dysfunction in relation with thyroid functions a in tertiary care hospital. Methods: 100 patients of CLD were included.Blood sample was analysed for Thyroid profile (TSH, FT4, FT3, total T3 and rT3), Serum LFT, Serum creatinine, Prothrombin time, INR andUSG of Hepatobiliary apparatus. Results were analysed to find out the correlation of thyroid profile with the severity of liver dysfunctionaccording to Child–Turcotte–Pugh (CTP) score and Model for End-Stage Liver Disease (MELD) score. Results: Majority of the patient had lowFT3 (90 %), Normal FT4 (58%), Normal TT3 (57%), high rT3 (57 %), and a normal TSH (72%). High TSH, low FT4, low FT3, low Tota l T3(TT3) and high rT3 correlated with the severity of liver disease as per CTP score. High TSH, low FT3 and low total T3 correlated with theMELD score. Conclusion: This study showed the existence of several abnormalities in thyroid function test in CLD, although euthyroidism wasmaintained in the majority. Low FT3 and low TT3 might be used as a predictor of severity in CLD