Biased sampling techniques

R. L. C. Vink, S. Wolfsheimer, and T. Schilling (2005) Isotropic-nematic interfacial tension of hard and soft rods Application of advanced grand canonical biased-sampling techniques. J. Chem. Phys. 123, 074901... 

Umbrella sampling is a biased-sampling technique developed by Torrie and Valleau [26] that enhances the sampling by modifying the potential energy function, forcing the system to stay in the desired states. The basic idea of umbrella sampling is presented below for further details, one can refer to the recent review paper by Kastner [27]. 

The biased-sampling approach may be considerably generalized, to allow the construction of MC moves step-by-step, with each step depending on the success or failure of the last. Such a procedure is biased, but it is then possible to correct for the bias (by considering the possible reverse moves). The technique has dramatically speeded up polymer simulations, and is capable of wider application. 

Speleothem frequency distributions have provided a useful tool for broad comparisons, but they suffer from the problem of biased sampling strategies and low resolution at times of known abrupt change. The increased precision afforded by mass-spectrometric techniques will result in fewer studies using this approach to assess of growth frequency and, more often, records of continuous deposition and growth rate studies will be graphically illustrated. 

Thus far, we have been discussing the crystallization of a multichain system. However, under suitable conditions, crystallization can even occur in a single-chain system. Using a combination of biased sampling, multihistogram techniques, and parallel tempering [ 125], we can directly compute the... 

Several round-robin intercalibrations for trace metals in seawater [26-30] have demonstrated a marked improvement in both analytical precision and numerical agreement of results among different laboratories. However, it has often been claimed that spurious results for the determination of metals in seawater can arise unless certain sampling devices and practical methods of sampler deployment are applied to the collection of seawater samples. It is therefore desirable that the biases arising through the use of different, commonly used sampling techniques be assessed to decide upon the most appropriate technique ) for both oceanic baseline and nearshore pollution studies. 

If the material comes in containers, or can be viewed as discrete units, the objective may be to estimate the number of units outside of specification. The value of the estimate is largely dependent on the sampling technique adopted. It is of little value, and could impart false information, if it is biased or imprecise. 

T. C. Beutler and W. F. van Gunsteren,/. Chem. Phys., 100,1492 (1994). The Computation of a Potential of Mean Force Choice of the Biasing Potential in the Umbrella Sampling Technique. 

The selection of samples, or where to sample, is the easier of the two problems. The selection of samples should not be left to the convenience of the operator as the convenient sample is usually heavily biased in a segregated powder. The sample at the top of the drum or first out of the mixer is not likely to be representative of the whole. The use of random numbers to identify sample locations is exemplified in section 2.3 and this technique removes the chance of operator selection bias. The systematic sample technique is useful as a routine industrial technique and involves taking a sample at regular time or space intervals. Care has to be exercised not to select an interval which coincides with a process interval such as batch size or shift change. 

The basic techniques of the conventional Monte Carlo methods applied to fluid problems have changed little since their invention. Recently, however, there has been a good deal of experimentation seeking to tinker with the technique in order to get new kinds of information. This chapter examined a (no doubt biased) sample of such attempts. 

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