Strong electronic correlation in the hydrogen chain: A variational Monte Carlo study

In this paper, we report a fully ab initio variational Monte Carlo study of the linear and periodic chain of hydrogen atoms, a prototype system providing the simplest example of strong electronic correlation in low dimensions. In particular, we prove that numerical accuracy comparable to that of benchmark density-matrix renormalization-group calculations can be achieved by using a highly correlated Jastrow-antisymmetrized geminal power variational wave function. Furthermore, by using the so-called "modern theory of polarization" and by studying the spin-spin and dimer-dimer correlations functions, we have characterized in detail the crossover between the weakly and strongly correlated regimes of this atomic chain. Our results show that variational Monte Carlo provides an accurate and flexible alternative to highly correlated methods of quantum chemistry which, at variance with these methods, can be also applied to a strongly correlated solid in low dimensions close to a crossover or a phase transition.

Predator cue studies reveal strong trait-mediated effects in communities despite variation in experimental designs

Nonconsumptive or trait-mediated effects of predators on their prey often outweigh density-mediated interactions where predators consume prey. For instance, predator presence can alter prey behaviour, physiology, morphology and/or development. Despite a burgeoning literature, our ability to identify general patterns in prey behavioural responses may be influenced by the inconsistent methodologies of predator cue experiments used to assess trait-mediated effects. We therefore conducted a meta-analysis to highlight variables (e.g. water type, predator husbandry, exposure time) that may influence invertebrate prey's behavioural responses to fish predator cues. This revealed that changes in prey activity and refuge use were remarkably consistent overall, despite wide differences in experimental methodologies. Our meta-analysis shows that invertebrates altered their behaviour to predator cues of both fish that were fed the focal invertebrate and those that were fed other prey types, which suggests that invertebrates were not responding to specific diet information in the fish cues. Invertebrates also altered their behaviour regardless of predator cue addition regimes and fish satiation levels. Cue intensity and exposure time did not have significant effects on invertebrate behaviour. We also highlight that potentially confounding factors, such as parasitism, were rarely recorded in sufficient detail to assess the magnitude of their effects. By examining the likelihood of detecting trait-mediated effects under large variations in experimental design, our study demonstrates that trait-mediated effects are likely to have pervasive and powerful influences in nature.