MC-SPEC: Specify how to do the Markov chain simulation. MC-spec writes records to a log file that specify the operations making up a single Markov chain iteration, and that also specify how dynamic trajectories are to be computed. When invoked with just a log file as argument, it displays the specifications stored at the given index, or the last specifications stored if no index is given. MC-spec is often invoked several times during a single simulation run. The last specification in the log file is used for further iterations. Usage: mc-spec log-file { operation-spec } [ / trajectory-spec ] or: mc-spec log-file [ index | "all" ] The log file must already exist. For the first form, specifications are appended to the log file with index equal that of the last record in the file, or 0 if the log file has no non-negative records. For the second form, with no index specified, the last specifications stored in the log file are displayed. If an index is specified in the second form, the specifications in the record with the given index are displayed. If the string "all" (no quotes) is given after the log file, specifications at all indexes are displayed. The allowed operations can be grouped in the categories general, metropolis, dynamical, tempering, and slicing: General operations: repeat times Repeat the following operations the specified number of times. The repeated group of operations is terminated by 'end', or the end of the list. end Terminates a group of operations. plot Prints the current values of those quantities specified in the "xxx-mc" command on standard output, preceded by an index that is reset to zero every iteration. (any operation not otherwise defined here) [ number [ number ] ] Invoke the application-specific update procedure, passing the name of the operation, and the numerical parameters following it, which default to zero. Metropolis operations: metropolis [ stepsize-adjust[:stepsize-alpha] ] Do a simple Metropolis update, to all components at once, using a Gaussian proposal distribution. met-1 [ stepsize-adjust[:stepsize-alpha] [ first[:last] ] ] Do single-component Metropolis updates, for the range of components given. The default if no components are given is all components. The default for last if first alone is given is last=first. Dynamical operations: heatbath [ decay ] Do a heatbath update of the momentum variables. If decay is zero (the default), the current momentum is forgotten, and new values are picked randomly from their distribution. If decay is non-zero, the momentum variables are multiplied by decay, and Gaussian noise with variance 1-decay^2 is then added. radial-heatbath Do a radial heatbath update of the momentum variables, in which the squared magnitude of the momentum is sampled from a chi-squared distribution, while the direction is left unchanged. negate Negate the momentum variables. dynamic steps [ stepsize-adjust[:stepsize-alpha] ] Follow a dynamical trajectory for the given number of steps, always accepting the result (hence does not leave distribution exactly invariant). permuted-dynamic steps [ stepsize-adjust[:stepsize-alpha] ] Like 'dynamic', but the order of approximations is randomly permuted. hybrid steps[:window[:jump]] [ stepsize-adjust[:stepsize-alpha] ] OR hybrid max-steps/max-ok[:jump] [ stepsize-adjust[:stepsize-alpha] ] Use the results of following a dynamical trajectory as a candidate state for a Metropolis update. There are two forms, differing in the way length of a trajectory and its acceptance are determined. In both forms, states along a trajectory are looked at only at every 'jump' steps (and either 'steps' or 'max-steps' must be multiples of 'jump'). The default is a jump of one. The stepsize is determined as for the 'dynamic' operation. In both forms, the momentum is negated if the proposal is accepted in order to make the step reversible. In the first form, acceptance is based on a 'window' of states at the beginning and end of the trajectory. In the second form, acceptance is based on a single state, but the number of steps in the trajectory is not fixed - instead, the trajectory ends after max-ok states that would be accepted have been found (looking only every 'jump' states), or when max-steps states of any sort have been produced; if the trajectory ends for the first reason, acceptance is guaranteed. The default if neither 'max-ok' nor 'window' is specified is standard hybrid Monte Carlo (ie, a window of one). tempered-hybrid temper-factor steps[:window[:jump]] [ stepsize-adjust[:stepsize-alpha] ] Like hybrid, but the trajectory is "tempered" by multiplying the momenta by temper-factor in the first half of the portion of the trajectory outside any windows, and by 1/temper-factor in the second half. spiral temper-factor steps [ stepsize-adjust[:stepsize-alpha] ] double-spiral temper-factor steps [ stepsize-adjust[:stepsize-alpha] ] Slicing operations: slice-1 [ stepsize-adjust[:stepsize-alpha] [ max-steps [ first[:last] ] ] ] Do single-component slice sampling, for the range of components given. The default for the range of components (first:last) is all components. The default for last if first alone is given is last=first. Components are numbered starting with zero. If the max-steps argument is positive, it gives the maximum number of intervals to create when using the stepping out procedure to find an interval around the current point. A value of zero indicates that stepping out should be done with no limit (this is the default). If max-steps is negative, the interval is found by doubling, with minus max-steps again being the limit on the number of intervals (so setting max-steps to -1 gives the same results as setting it to 1). (There is no way to double without limit, but setting max-steps to -1000 is the same for all practical purposes.) slice-over [ refinements [ refresh-prob [ stepsize-adjust[:stepsize-alpha] [ max-steps [ first[:last] ] ] ] ] ] Do overrelaxed slice sampling, for the range of components given. The endpoints are computed using the given number of refinements (default zero). The refresh probability is the probability of doing an ordinary slice sampling update rather than an overrelaxed one; it defaults to zero. The default range of components if no range is given is all components. The default for last if first alone is given is last=first. The meaning of max-steps is the same as for slice-1. slice-inside steps [ stepsize-adjust[:stepsize-alpha] ] Performs multivariate slice sampling by reflection from inside points, proceeding for the indicated number of steps, with the indicated stepsize adjustment factor. The momentum is negated in such a way as to make the operation reversible. slice-outside steps[/in-steps] [ stepsize-adjust[:stepsize-alpha] ] Performs multivariate slice sampling by reflection from outside points, proceeding for the indicated number of steps, with the indicated stepsize adjustment factor. The momentum is negated at the end in such a way as to make the operation reversible. The in-steps argument gives a limit on the number of steps that can be inside the slice; it defaults to the same as steps (thereby having no effect). Setting in-steps to less than steps may decrease the chances of the trajectory ending on an outside point, and being rejected. Tempering/annealing operations: See also the descriptions under "dynamical operations" of the spiral, double-spiral, and tempered-hybrid operations. sim-temp Do a metropolis update of the simulated tempering inverse temperature, with a proposal of changing the temperature index in accord with the current tempering direction. The direction is negated if the proposal is accepted to make the step reversible. rand-dir Randomize the tempering direction. neg-dir Negate the tempering direction. temp-trans Perform a tempered transition, with components given by the following operations (terminated by 'end'). The components are done in forward order for the first half of the tempered transition, in reverse order for the second. For this to work correctly, the components must be reversible in themselves. AIS Proceed to the next step of an annealed importance sampling run, with the tempering index increased by one, adjusting the importance weight appropriately. If the tempering index is already at the final value (inverse temperature of one), a new state is randomly generated from the distribution at inverse temperature zero, and the tempering index is set to the beginning of the tempering schedule. Test operations: The following operations are used for testing and research only. They do not leave the desired distribution invariant, and hence should not be used as part of a sampling scheme. multiply-momentum factor All momentum variables are multiplied by the given factor. set-momentum value All momentum variables are set to the given value. Certain of these operations are normally used in standard combinations, in particular the following: heatbath hybrid <steps> Standard hybrid Monte Carlo heatbath <decay> hybrid <steps> negate Persistent Hybrid Monte Carlo rand-dir sim-temp Standard simulated tempering sim-temp neg-dir Persistent Simulated tempering All operations are reversible (other than 'ais', 'repeat', and 'end' for which the concept is not applicable), except for 'dynamic', 'permuted-dynamic', 'multiply-momentum', 'set-momentum', and perhaps the application-specific operations. However, note that in general sequential combinations of reversible operations are not reversible. Defaults are decay of zero, stepsize-adjust of one, stepsize-alpha of infinity, window of one, and jump of one. A minus sign before a stepsize-adjust value results in the adjustment being applied to uniform stepsizes of one; otherwise, the adjustment is applied to the application-specific stepsizes (which may be non-uniform). A minus sign in front of stepsize alpha means that instead of the usual Gamma distribution for stepsizes, a distribution over 'alpha' orders of magnitude, uniform in the log domain, centred at the value found using stepsize-adjust, is used. The trajectory specification can have one of the following forms: leapfrog [ halfp | halfq ] [ N-approx ] Use the leapfrog method, repeated N-approx times using a randomly selected ordering of that number of energy approximations (whose average must be the true value). The default for N-approx is one. The "halfp" or "halfq" option specifies whether the half-steps at the beginning and end of the trajectory are for p (momentum) or q (position). The default is "halfp". opt2 [ rev | sym ] [ firstq | firstp ] Apply McLachlan and Atela's "optimal" two stage method, with the first step being done for the position (firstq) or momentum (firstp) variables. The default is "firstq". This method is not symmetrical. The "rev" option gives the reversed version. The "sym" option gives the ordinary followed by the reversed version, with half the stepsize for each, thereby producing a symmetrical method. Only the symmetrical method should be used in conjuntion with "hybrid" and "tempered hybrid" operations. gen2 [ rev | sym ] [ firstq | firstp ] a2 Apply a two stage second-order method with parameter a2, as described by McLachlan and Atela. Firstq and firstp are as for opt2. This is probably only interesting for research, since the most useful values for a2 are covered by "leapfrog" and "opt2". The "rev" and "sym" options are as for opt2. opt4 Apply McLachlan and Atela's "optimal" four stage method for quadratic kinetic energy. The "rev" and "sym" options are as for opt2. The default if no specification is given is "leapfrog halfp 1". The opt2, gen2, and opt4 methods are taken from a paper by Robert I. McLachlan and Pau Atela, "The accuracy of symplectic integrators", Nonlinearity, vol. 5, pp. 541-562, 1992. (Note: there is a typo in their description of opt2 in table 2. b1=1-b2 should be 1-1/sqrt(2).) The list of operations is stored in a record of type 'o'; the trajectory specification in a record of type 't'. Copyright (c) 1995, 1998, 1999 by Radford M. Neal