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Results from life cycle assessment (LCA) studies are sensitive to modeling choices and data used in building the underlying model. This is also relevant for the case of fisheries and LCAs of fish products. Fisheries' product systems show both multifunctionality because of the simultaneous co‐catch of multiple species and potential constraints to supply due to natural stock limits or socially established limits such as quota systems. The performance of fisheries also varies across seasons, locations, vessels, and target species. In this study, we investigate the combined effect of modeling choices and variability on the uncertainty of LCA results of fish products. We use time series data from official Danish statistics for catch and fuel use of several fisheries disaggregated using a top‐down procedure. We apply multiple modeling approaches with different assumptions regarding the type of partitioning, substitution, and constraints. The analysis demonstrates that, in the presence of relevant multifunctionality, the results are substantially affected by the modeling approach chosen. These findings are robust across years and fisheries, indicating that modeling choices contribute to uncertainty more than the variability in fishing conditions. We stress the need for a more careful alignment of research questions and methods for LCA studies of fisheries and recommend a very transparent statement of assumptions, combined with uncertainty and sensitivity analysis. This article met the requirements for a gold‐gold data openness badge described at http://jie.click.badges. [ABSTRACT FROM AUTHOR]

The first implementation of tensor-decomposed vibrational coupled cluster (CP-VCC) response theory for calculating vibrational excitation energies is presented. The CP-VCC algorithm, which has previously been applied to solving the vibrational coupled cluster (VCC) ground-state equations without explicitly constructing any tensors of order three or higher, has been generalized to allow transformations with the Jacobian matrix necessary for computation of response excitation energies by iterative algorithms. A new eigenvalue solver for computing CP-VCC excitation energies is introduced, and the different numerical thresholds used for controlling the accuracy of the obtained eigenvalues are discussed. Numerical results are presented for calculations of the 20 lowest eigenvalues on a set of 10 four-atomic molecules, as well as for a number of polycyclic aromatic hydrocarbons (PAHs) of increasing size, up to PAH8 with 120 modes. It is shown that the errors introduced by the tensor decomposition can be controlled by the choice of numerical thresholds. Furthermore, all thresholds can be defined relative to the requested convergence threshold of the equation solver, which allows black-box calculations with minimal user input to be performed. Eigenstates of PAHs were efficiently computed without any explicitly constructed tensors, showing improvements in both memory and central processing unit time compared to the existing full-tensor versions. [ABSTRACT FROM AUTHOR]

The first general excitation level implementation of the time-dependent vibrational coupled cluster (TDVCC) method introduced in a recent publication [J. Chem. Phys. 151, 154116 (2019)] is presented. The general framework developed for time-independent vibrational coupled cluster (VCC) calculations has been extended to the time-dependent context. This results in an efficient implementation of TDVCC with general coupling levels in the cluster operator and Hamiltonian. Thus, the convergence of the TDVCC[k] hierarchy toward the complete-space limit can be studied for any sum-of-product Hamiltonian. Furthermore, a scheme for including selected higher-order excitations for a subset of modes is introduced and studied numerically. Three different definitions of the TDVCC autocorrelation function (ACF) are introduced and analyzed in both theory and numerical experiments. Example calculations are presented for an array of systems including imidazole, formyl fluoride, formaldehyde, and a reduced-dimensionality bithiophene model. The results show that the TDVCC[k] hierarchy converges systematically toward the full-TDVCC limit and that the implementation allows accurate quantum-dynamics simulations of large systems to be performed. Specifically, the intramolecular vibrational-energy redistribution of the 21-dimensional imidazole molecule is studied in terms of the decay of the ACF. Furthermore, the importance of product separability in the definition of the ACF is highlighted when studying non-interacting subsystems. [ABSTRACT FROM AUTHOR]

We develop time-dependent vibrational coupled cluster with time-dependent modals (TDMVCC), where an active set of one-mode basis functions (modals) is evolved in time alongside coupled-cluster wave-function parameters. A biorthogonal second quantization formulation of many-mode dynamics is introduced, allowing separate biorthogonal bases for the bra and ket states, thus ensuring complex analyticity. We employ the time-dependent bivariational principle to derive equations of motion for both the one-mode basis functions and the parameters describing the cluster (T) and linear de-excitation (L) operators. The choice of constraint (or gauge) operators for the modal time evolution is discussed. In the case of untruncated cluster expansion, the result is independent of this choice, but restricting the excitation space removes this invariance; equations for the variational determination of the constraint operators are derived for the latter case. We show that all single-excitation parts of T and L are redundant and can be left out in the case of variationally determined constraint-operator evolution. Based on a pilot implementation, test computations on Henon–Heiles model systems, the water molecule, and a reduced-dimensionality bi-thiophene model are presented, showing highly encouraging results for TDMVCC. It is demonstrated how TDMVCC in the limit of a complete cluster expansion becomes equivalent to multiconfiguration time-dependent Hartree for the same active-space size. Similarly, it is discussed how TDMVCC generally gives better and more stable results than its time-independent-modals counterpart, while equivalent results are obtained for complete expansions and full one-mode basis sets. [ABSTRACT FROM AUTHOR]

For the first time, equations are derived for computing stationary vibrational states with extended vibrational coupled cluster (EVCC) and for propagating nuclear wave packets using time-dependent EVCC (TDEVCC). Expressions for energies, properties, and auto-correlation functions are given. For TDEVCC, convergence toward the ground state for imaginary-time propagation is shown, as well as separability in the case of non-interacting subsystems. The analysis focuses substantially on the difference between bra and ket parameterizations for EVCC and TDEVCC compared to normal vibrational coupled cluster (VCC) and time-dependent VCC (TDVCC). A pilot implementation is presented within a new full-space framework that offers easy access to completely general, albeit not efficient, implementations of alternative VCC variants, such as EVCC. The new methods were tested on 35 three- and six-mode molecular systems. Both EVCC[k] and TDEVCC[k] showed good, hierarchical convergence toward the exact limit. This convergence was generally better than for normal VCC[k] and TDVCC[k] and better still than for (time-dependent) vibrational configuration interaction, though this should be balanced with the higher computational complexity of EVCC. The results highlight the importance of exponential parameterizations and separability in general, as seen, in particular, for the TDEVCC bra parameterization, which is in contrast to the partially linear one of TDVCC. With the results being rooted in the general structures of coupled cluster (CC) theory, they are expected to be relevant to other applications of both normal and extended CC theory as well. [ABSTRACT FROM AUTHOR]

The multiconfiguration time-dependent Hartree (MCTDH) method is a powerful method for solving the time-dependent Schrödinger equation in quantum molecular dynamics. It is, however, hampered by the so-called curse of dimensionality which results in exponential scaling with respect to the number of degrees of freedom in the system and, thus, limits its applicability to small- and medium-sized molecules. To avoid this scaling, we derive equations of motion for a series of truncated MCTDH methods using a many-mode second-quantization formulation where the configuration space is restricted based on mode-combination levels as also done in the vibrational configuration interaction and vibrational coupled cluster methods for solving the time-independent Schrödinger equation. The full MCTDH wave function is invariant with respect to the choice of constraint (or gauge) operators, but restricting the configuration space removes this invariance. We, thus, analyze the remaining redundancies and derive equations for variationally optimizing the non-redundant matrix elements of the constraint operators. As an alternative, we also present a constraint that keeps the density matrices block diagonal during the propagation and the two choices are compared. Example calculations are performed on formyl fluoride and a series of high-dimensional Henon–Heiles potentials. The results show that the MCTDH[n] methods can be applied to large systems and that an optimal choice of constraint operators is key to obtaining the correct physical behavior of the wave function. [ABSTRACT FROM AUTHOR]

Equations are derived for the time evolution of time-dependent vibrational coupled cluster (TDVCC) wave functions covering both the TDVCC ket state and the associated so-called Λ bra state. The equations are implemented in the special case of both the Hamiltonian and the cluster operator containing at most two-mode coupling terms. The nontrivial behavior of the evolution of norm, energy, and expectation values due to the nonunitary time-evolution of the nonvariational TDVCC theory is analyzed theoretically and confirmed in numerical experiments that also include time-dependent Hamiltonians. In the spirit of time-independent size-consistency analysis, the separability of both the coupled cluster and Λ states for noninteracting systems is studied. While the coupled cluster state clearly has the correct behavior, the behavior of the Λ state is more intricate, and the consequence for different properties is shown theoretically and numerically. Overall, the numerical experiments show that TDVCC in incomplete expansions gives higher accuracy than a standard linear variational wave function parameterization with the same number of independent parameters, while equivalent results are obtained for complete expansions. The efficiency of the methodology is illustrated in computations on polycyclic aromatic hydrocarbons with up to 156 modes. [ABSTRACT FROM AUTHOR]