X-ray crystallography provides the vast majority of macromolecular structures, but the success of the method relies on growing crystals of sufficient size. In conventional measurements, the necessary increase in X-ray dose to record data from crystals that are too small leads to extensive damage before a diffraction signal can be recorded1-3. It is particularly challenging to obtain large, well-diffracting crystals of membrane proteins, for which fewer than 300 unique structures have been determined despite their importance in all living cells. Here we present a method for structure determination where single-crystal X-ray diffraction ‘snapshots’ are collected from a fully hydrated stream of nanocrystals using femtosecond pulses from a hard-X-ray free-electron laser, the Linac Coherent Light Source4. We prove this concept with nanocrystals of photosystem I, one of the largest membrane protein complexes5. More than 3,000,000 diffraction patterns were collected in this study, and a three-dimensional data set was assembled from individual photosystem I nanocrystals (~200 nm to 2 μm in size). We mitigate the problem of radiation damage in crystallography by using pulses briefer than the timescale of most damage processes6. This offers a new approach to structure determination of macromolecules that do not yield crystals of sufficient size for studies using conventional radiation sources or are particularly sensitive to radiation damage.

Quantum Monte Carlo methods have proved very valuable to study the structure and reactions of light nuclei and nucleonic matter starting from realistic nuclear interactions and currents. These ab-initio calculations reproduce many low-lying states, moments and transitions in light nuclei, and simultaneously predict many properties of light nuclei and neutron matter over a rather wide range of energy and momenta. We review the nuclear interactions and currents, and describe the continuum Quantum Monte Carlo methods used in nuclear physics. These methods are similar to those used in condensed matter and electronic structure but naturally include spin-isospin, tensor, spin-orbit, and three-body interactions. We present a variety of results including the low-lying spectra of light nuclei, nuclear form factors, and transition matrix elements. We also describe low-energy scattering techniques, studies of the electroweak response of nuclei relevant in electron and neutrino scattering, and the properties of dense nucleonic matter as found in neutron stars. A coherent picture of nuclear structure and dynamics emerges based upon rather simple but realistic interactions and currents.

We report quantum Monte Carlo calculations of superfluid Fermi gases with short-range two-body attractive interactions with infinite scattering length. The energy of such gases is estimated to be (0.44 ± 0.01) times that of the noninteracting gas, and their pairing gap is approximately twice the energy per particle. PACS: 03.75.Fi, 05.30.Fk, 21.65.+F In dilute Fermi gases the pair interactions have a range much smaller than the interparticle spacing. However, when the two-particle scattering length is large, these short range interactions can modify the gas properties significantly. A well known example is low density neutron matter which may occur in the inner crust of neutron stars [1]. The two-neutron interaction has a range of ∼ 2 fm, but the scattering length is large, −18 fm, so that even at densities as small as one percent of the nuclear density the parameter ak F has magnitude much larger than one. Bertsch proposed in 1998 that solution of the idealized problem of a dilute Fermi gas in the limit ak F → −∞ could give useful insights into the properties of low density neutron gas.Cold dilute gases of 6 Li atoms have been produced in atom traps. The interaction between these atoms can be tuned using a known Feshbach resonance; and the estimated value of ak F in the recent experiment [2] is ∼ −7.4. As the interaction strength is increased beyond that for a = −∞, we get bosonic two-fermion bound states. In this sense a dilute Fermi gas with large a is in between weak coupling BCS superfluid and dilute Bose gases with Bose-Einstein condensation [3]. Attempts to produce Bose gases in the limit, a/r 0 → ∞ using Feshbach resonances [4,5], are in progress, and their energy has been recently estimated using variational methods [6].In the a → −∞ limit k 2 F /m is the only energy scale, and the ground state energy of the interacting dilute Fermi gas is proportional to the noninteracting Fermi gas energy:Baker [7] and Heiselberg [8] have attempted to obtain the value of the constant ξ from expansions of the Fermi gas energy in powers of ak F . Heiselberg obtained ξ = 0.326, while Baker's values are ξ = 0.326 and 0.568. Fermi gases with attractive pair interaction become superfluid at low temperature. The BCS expressions in terms of the scattering length were given by Leggett [9], and they were used to study the properties of superfluid dilute Fermi gases, as a function of ak F , by Engelbrecht, Randeria and Sá de Melo [10]. For ak F = −∞ they obtain an upperbound, ξ = 0.59, using the BCS wave function. These gases are also estimated to have large gaps comparable to the ground state energy per particle.Here we report studies of Fermi gases with quantum Monte Carlo methods using the model potential:The zero energy solution of the two-body Schrödinger equation with this potential is tanh(µr)/r and corresponds to a = −∞. The effective range is 2/µ, and in order to ensure that the gas is dilute we use µr 0 > 10, where r 0 is the unit radius; ρr 3 0 = 3/4π. All the results presented here are for µr 0 = 12; however some ...

As shown by Gañán-Calvo and co-workers, a free liquid jet can be compressed in diameter through gas-dynamic forces exerted by a co-flowing gas, obviating the need for a solid nozzle to form a microscopic liquid jet and thereby alleviating the clogging problems that plague conventional droplet sources of small diameter. We describe in this paper a novel form of droplet beam source based on this principle. The source is miniature, robust, dependable, easily fabricated, and eminently suitable for delivery of microscopic liquid droplets, including hydrated biological samples, into vacuum for analysis using vacuum instrumentation. Monodisperse, single file droplet streams are generated by triggering the device with a piezoelectric actuator. The device is essentially immune to clogging.

We present quantum Monte Carlo calculations of light nuclei, neutron-α scattering, and neutron matter using local two-and three-nucleon (3N) interactions derived from chiral effective field theory up to next-to-next-to-leading order (N 2 LO). The two undetermined 3N low-energy couplings are fit to the 4 He binding energy and, for the first time, to the spin-orbit splitting in the neutron-α P -wave phase shifts. Furthermore, we investigate different choices of local 3N-operator structures and find that chiral interactions at N 2 LO are able to simultaneously reproduce the properties of A = 3, 4, 5 systems and of neutron matter, in contrast to commonly used phenomenological 3N interactions.Three-nucleon (3N) interactions are essential for a reliable prediction of the properties of light nuclei and nucleonic matter [1][2][3][4][5]. In quantum Monte Carlo (QMC) calculations phenomenological 3N interactions such as the Urbana [6] and Illinois [7] models have been used with great success [3,8]. However, such models suffer from certain disadvantages: They are not based on a systematic expansion and it was found that the Illinois forces tend to overbind neutron matter [9,10]. It is therefore unlikely that these phenomenological models can be used to correctly predict the properties of heavy neutron-rich nuclei.An approach which addresses these shortcomings is chiral effective field theory (EFT) [2,[11][12][13][14]. Chiral EFT is a low-energy effective theory consistent with the symmetries of quantum chromodynamics and provides a systematic expansion for nuclear forces. It includes contributions from long-range pion-exchange interactions explicitly and expands the short-distance interactions into a systematic set of contact operators accompanied by low-energy couplings fit to experimental data. Chiral EFT enables the determination of theoretical uncertainties and systematic order-by-order improvement; for recent work see Refs. [15][16][17][18].Chiral EFT also predicts consistent many-body interactions. In Weinberg power counting, 3N forces first enter at next-to-next-to-leading order (N 2 LO) [19,20] and contain three contributions: A two-pion-exchange interaction V C , a one-pion-exchange-contact interaction V D , and a 3N contact interaction V E . While the first is accompanied by the couplings c i from the pion-nucleon sector, the latter two are accompanied by the couplings c D and c E , which have to be determined in A > 2 systems.In addition to systematic nuclear forces, reliable manybody methods are required to describe properties of light nuclei and of dense neutron matter. QMC approaches, which solve the many-body Schrödinger equation stochastically, are such a class of methods. Both the Green's function Monte Carlo (GFMC) method and the auxiliary-field diffusion Monte Carlo (AFDMC) method rely on projection in imaginary time τ ,with H the Hamiltonian of the system and |Ψ T a trial wave function not orthogonal to the many-body ground state |Ψ 0 . For a recent review of developments and applications of QMC methods...

Photosynthesis, a process catalysed by plants, algae and cyanobacteria converts sunlight to energy thus sustaining all higher life on Earth. Two large membrane protein complexes, photosystem I and II (PSI and PSII), act in series to catalyse the light-driven reactions in photosynthesis. PSII catalyses the light-driven water splitting process, which maintains the Earth’s oxygenic atmosphere1. In this process, the oxygen-evolving complex (OEC) of PSII cycles through five states, S0 to S4, in which four electrons are sequentially extracted from the OEC in four light-driven charge-separation events. Here we describe time resolved experiments on PSII nano/microcrystals from Thermosynechococcus elongatus performed with the recently developed2 technique of serial femtosecond crystallography. Structures have been determined from PSII in the dark S1 state and after double laser excitation (putative S3 state) at 5 and 5.5 Å resolution, respectively. The results provide evidence that PSII undergoes significant conformational changes at the electron acceptor side and at the Mn4CaO5 core of the OEC. These include an elongation of the metal cluster, accompanied by changes in the protein environment, which could allow for binding of the second substrate water molecule between the more distant protruding Mn (referred to as the ‘dangler’ Mn) and the Mn3CaOx cubane in the S2 to S3 transition, as predicted by spectroscopic and computational studies3,4. This work shows the great potential for time-resolved serial femtosecond crystallography for investigation of catalytic processes in biomolecules.

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Articles you may be interested inCorrelation consistent basis sets for molecular core-valence effects with explicitly correlated wave functions: The atoms B-Ne and Al-Ar Accuracy of electronic wave functions in quantum Monte Carlo: The effect of high-order correlationsWe apply the variational Monte Carlo method to the atoms He through Ne. Our trial wave function is of the form introduced by Boys and Handy. We use the Monte Carlo method to calculate the first and second derivatives of an unreweighted variance and apply Newton's method to minimize this variance. We motivate the form ofthe correlation function using the local current conservation arguments of Feynman and Cohen. Using a self-consistent field wave function multiplied by a Boys and Handy correlation function, we recover a large fraction ofthe correlation energy of these atoms. We give the value of all variational parameters necessary to reproduce our wave functions. The method can be extended easily to other atoms and to molecules. 4172