Two algorithms for the local energy minimization of the structure of macromolecules in the presence of constraints are proposed. They are a combination of the method of steepest descents and the method of conjugate gradients with the procedure SHAKE, by which distance constraints can be satisfied. The two algorithms are tested by applying them to a small protein, the bovine pancreatic trypsin inhibitor (BPTI), and compared with the penalty function method for conserving constraints. The efficiency of the proposed methods depends on the level of interdependence of the constraints. For bond-length constraints, the use of SHAKE is superior to the penalty function method. However, when bond-angle constraints are included, SHAKE is more efficient only if the curvature of the penalty function is considerably greater than that of the potential function being minimized. The results indicate that with bond-length constraints the minimization behavior is similar to that without constraints. However, the simultaneous application of bond-length and bond-angle constraints appears to confine the molecule to a very limited part of configuration space, very different from the part covered by an unconstrained minimization. This conclusion calls into question energy minimizations of protein systems in which only the dihedral angles are allowed to vary. 相似文献
A comparison of different treatments of bond-stretching interactions in molecular dynamics simulation is presented. Relative free energies from simulations using rigid bonds maintained with the SHAKE algorithm, using partially rigid bonds maintained with a recently introduced flexible constraints algorithm, and using fully flexible bonds are compared in a multi-configurational thermodynamic integration calculation of changing liquid water into liquid methanol. The formula for the free energy change due to a changing flexible constraint in a flexible constraint simulation is derived. To allow for a more direct comparison between these three methods, three different pairs of models for water and methanol were used: a flexible model (simulated without constraints and with flexible constraints), a rigid model (simulated with standard hard constraints), and an alternative flexible model (simulated with flexible constraints and standard hard constraints) in which the ideal or constrained bond lengths correspond to the average bond lengths obtained from a short simulation of the unconstrained flexible model. The particular treatment of the bonds induces differences of up to 2 % in the liquid densities, whereas (excess) free energy differences of up to 5.7 (4.3) kJ mol(-1) are observed. These values are smaller than the differences observed between the three different pairs of methanol/water models: up to 5 % in density and up to 8.5 kJ mol(-1) in (excess) free energy. 相似文献
Some of the pitfalls that may befall molecular simulations of interfaces are discussed. They are all related to the calculation of the pressure tensor profiles, which are needed in order to compute surface tensions. We focus on three controversial points: (1) the calculation of the pressure tensor profiles for polyatomic systems, in particular, when the SHAKE algorithm is employed, (2) the addition of long-range corrections to compensate the truncation of the potential, and (3) the importance of including adequate error intervals with the results. Most of the conclusions are general, but some specifically apply to multiple site molecular-dynamics simulations. 相似文献
Summary: We have shown that the components of Cartesian rotation vectors can be used successfully as generalized coordinates describing angular orientation in Brownian dynamics simulations of non‐spherical nanoparticles. For this particular choice of generalized coordinates, we rigorously derived the conformation‐space diffusion equations from kinetic theory for both free nanoparticles and nanoparticles interconnected by springs or holonomic constraints into polymer chains. The equivalent stochastic differential equations were used as a foundation for the Brownian dynamics algorithms. These new algorithms contain singularities only for points in the conformation‐space where both the probability density and its first coordinate derivative equal zero (weak singularities). In addition, the coordinate values after a single Brownian dynamics time step are throughout the conformation‐space equal to the old coordinate values plus the respective increments. For some parts of the conformation‐space these features represent a major improvement compared to the situation when Eulerian angles describe rotational dynamics. The presented simulation results of the equilibrium probability density for free nanoparticles are in perfect agreement with the results from kinetic theory.
Simulation of p(eq)(Φ) for free nanoparticles. 相似文献
We have developed a time-reversible rigid-body (rRB) molecular dynamics algorithm in the isothermal-isobaric (NPT) ensemble. The algorithm is an extension of rigid-body dynamics [Matubayasi and Nakahara, J Chem Phys 1999, 110, 3291] to the NPT ensemble on the basis of non-Hamiltonian statistical mechanics [Martyna, G. J. et al., J Chem Phys 1994, 101, 4177]. A series of MD simulations of water as well as fully hydrated lipid bilayer systems have been undertaken to investigate the accuracy and efficiency of the algorithm. The rRB algorithm was shown to be superior to the state-of-the-art constraint-dynamics algorithm SHAKE/RATTLE/ROLL, with respect to computational efficiency. However, it was revealed that both algorithms produced accurate trajectories of molecules in the NPT as well as NVT ensembles, as long as a reasonably short time step was used. A couple of multiple time-step (MTS) integration schemes were also examined. The advantage of the rRB algorithm for computational efficiency increased when the MD simulation was carried out using MTS on parallel processing computer systems; total computer time for MTS-MD of a lipid bilayer using 64 processors was reduced by about 40% using rRB instead of SHAKE/RATTLE/ROLL. 相似文献
The four different schemes of Group Explicit Method (GEM):GER,GEL,SAGE and DAGE have been claimed to be unstable when employed for electrochemical digital simulations with large model diffusion coefficient DM.However,in this investigation,in spite of the conditional stability of GER and GEL,the SAGE scheme,which is a combination of GEL and GER,was found to be unconditionally stable when used for simulations of electrochemical reaction-diffusion and had a performance comparable with or even better than the fast Quasi Explicit Finite Difference Method(FQEFD) in some aspects.Corresponding differential equations of SAGE scheme for digital simulations of various electrochemical mechanisms with both uniform and exponentially expanded space units were established.The effectiveness of the SAGE method was further demonstrated by the simulations of an EC and a catalytic mechanism with very large homogeneous rate constants. 相似文献
A site‐specific Cu2+ binding motif within a DNA duplex for distance measurements by ESR spectroscopy is reported. This motif utilizes a commercially available 2,2′‐dipicolylamine (DPA) phosphormadite easily incorporated into any DNA oligonucleotide during initial DNA synthesis. The method only requires the simple post‐synthetic addition of Cu2+ without the need for further chemical modification. Notably, the label is positioned within the DNA duplex, as opposed to outside the helical perimeter, for an accurate measurement of duplex distance. A distance of 2.7 nm was measured on a doubly Cu2+‐labeled DNA sequence, which is in exact agreement with the expected distance from both DNA modeling and molecular dynamic simulations. This result suggests that with this labeling strategy the ESR measured distance directly reports on backbone DNA distance, without the need for further modeling. Furthermore, the labeling strategy is structure‐ and nucleotide‐independent. 相似文献
Studying the growth and stability of anisotropic or isotropic disordered surfaces in electrodeposition is of importance in catalytic electrochemistry. In some cases, the metallic nature of the electrode defines the topography and roughness, which are also controlled by the experimental time and applied external potential. Because of the experimental restrictions in conventional electrochemical techniques and ex situ electron microscopies, a theoretical model of the surface geometry could aid in understanding the electrodeposition process and current distributions. In spite of applying a complex theory such as dynamic scaling method or perturbation theories, the resolution of mixed mass‐/charge‐transfer equations (tertiary distribution) for the electrodeposition process would give reliable information. One of the main problems with this type of distribution is the mathematics when solving the spatial n‐dimensional differential equations. Use of a primary current distribution is proposed here to simplify the differential equations; however it limits wide application of the first assumption. Distributions of concentration profile, current density, and electrode potential are presented here as a function of the distance normal to the surface for the cases of smooth and rough platinum growth. In the particular case of columnar surfaces, cycloid curves are used to model the electrode, from which the concentration profile is presented in a parameterized form after solving a first‐type curvilinear integral. The concentration contour results in a combination of a trigonometric inverse function and a linear distribution leading to a negative concavity curve. The calculation of the current density and electrode potential contours also show trigonometric shapes exhibiting forbidden imaginary values only at the minimal values of the trochoid curve. 相似文献
Molecular dynamics (MD) simulations of human adult hemoglobin (HbA) were carried out for 45 ns in water with all degrees of freedom including bond stretching and without any artificial constraints. To perform such large-scale simulations, one of the authors (M.S.) accelerated his own software COSMOS90 on the Earth Simulator by vectorization and parallelization. The dynamical features of HbA were investigated by evaluating root-mean-square deviations from the initial X-ray structure (an oxy T-state hemoglobin with PDB code: 1GZX) and root-mean-square fluctuations around the average structure from the simulation trajectories. The four subunits (alpha(1), alpha(2), beta(1), and beta(2)) of HbA maintained structures close to their respective X-ray structures during the simulations even though no constraints were applied to HbA in the simulations. Dimers alpha(1)beta(1) and alpha(2)beta(2) also maintained structures close to their respective X-ray structures while they moved relative to each other like two stacks of dumbbells. The distance between the two dimers (alpha(1)beta(1) and alpha(2)beta(2)) increased by 2 A (7.4%) in the initial 15 ns and stably fluctuated at the distance with the standard deviation 0.2 A. The relative orientation of the two dimers fluctuated between the initial X-ray angle -100 degrees and about -105 degrees with intervals of a few tens of nanoseconds. 相似文献