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The combined density functional theory and multireference configuration interaction (DFT/MRCI) method is a powerful tool for the calculation of excited electronic states of large molecules. There exists, however, a large amount of superfluous configurations in a typical DFT/MRCI wave function. We show that this deadwood may be effectively removed using a simple configuration pruning algorithm based on second-order Epstein–Nesbet perturbation theory. The resulting method, which we denote p-DFT/MRCI, is shown to result in orders of magnitude saving in computational timings, while retaining the accuracy of the original DFT/MRCI method.

We report how imaginary time wavepacket propagation may be used to efficiently calculate the lowest-lying eigenstates of the electronic Hamiltonian. This approach, known as the relaxation method in the quantum dynamics community, represents a fundamentally different approach to the solution of the electronic eigenvalue problem in comparison to traditional iterative subspace diagonalization schemes such as the Davidson and Lanczos methods. In order to render the relaxation method computationally competitive with existing iterative subspace methods, an extended short iterative Lanczos wavepacket propagation scheme is proposed and implemented. In the examples presented here, we show that by using an efficient wavepacket propagation algorithm the relaxation method is, at worst, as computationally expensive as the commonly used block Davidson–Liu algorithm, and in certain cases, significantly less so.

AbstractThe ecology of estuaries on the south coast of Western Australia has been disrupted by increased nutrient and sediment discharge from predominantly rural catchments. A range of conventional management actions are available to reduce nutrient loss at source, and it is important to evaluate possible reductions, and costs, so that limited funds can be targeted to realise the greatest moderation of nutrient loss. A lumped landuse nutrient generation rate model was developed for four catchments (Wilson Inlet, Oyster Harbour, Torbay Inlet and Princess Royal Harbour) near Albany Western Australia and the output compared with existing monitoring data. The nutrient moderating effects of six conventional management actions (perennial pastures; vegetated stream buffers; minimum tillage, effective fertiliser use; stock control and water management; and effluent management) and their associated costs were implemented at different levels in the model. These were used to determine the extent to which these actions could address offsite nutrient pollution, and the cost of doing so. Management actions were implemented in three major scenarios representing the status quo (current nutrient reduction efforts through volunteerism), the highest possible implementation of each action (perhaps indicating mandated change) and the most cost effective set of actions. In each catchment dominated by diffuse nutrient sources, current nutrient reduction efforts amounted to about 10%, whilst the highest possible reductions were of the order of25–30% above this. In the point source dominated catchment current nutrient reduction efforts amounted to about 40%, with an additional 40% possible. The most cost effective scenarios reduced nitrogen more than phosphorus. Under the most cost effective scenarios, it was estimated that the net cost of management actions over 10 years was budget positive, resulting in a net benefit to the land managers involved. This suggests that there are limited economic barriers to the adoption of these conventional management actions, and on this basis current nutrient reduction efforts could be significantly increased. However, predicted reductions from the implementation of conventional management actions may not be sufficient to arrest estuarine decline.