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THE PARAMETER FILE DDSCAT.PAR
9.6
Maximum Number of Iterations
MXITER = maximum number of complex-conjugate-gradient iterations allowed. As there are 3N equations to solve, MXITER should never be larger than 3N , but in practice should be much smaller. As a default we suggest setting MXITER=100, but for some problems that converge very slowly you may need to use a larger value of MXITER.
9.7
Interaction Cutoff Parameter for PBC Calculations
GAMMA = parameter limiting certain summations that are required for periodic targets (Draine & Flatau 2008, see). The value of GAMMA has no effect on computations for finite targets – it can be set to any value, including 0). For targets that are periodic in 1 or 2 dimensions, GAMMA needs to be small for high accuracy, but the required cpu time increases as GAMMA becomes smaller. GAMMA = 10−2 is reasonable for initial calculations, but you may want to experiment to see if the results you are interested in are sensitive to the value of GAMMA. Draine & Flatau (2008) show examples of how computed results for scattering can depend on the value of γ.
9.8
Angular Resolution for Computation of hcos θi, etc.
The parameter ETASCA determines the selection of scattering angles used for computation of certain angular averages, such as hcos θi and hcos2 θi, and the radiation pressure force (see §16) and radiative torque (if CMDTRQ=DOTORQ). Small values of ETASCA result in increased accuracy but also cost additional computation time. ETASCA=0.5 generally gives accurate results. If accurate computation of hcos θi or the radiation pressure force is not required, the user can set ETASCA to some large number, e.g. 10, to minimize unnecessary computation.
9.9
Vacuum Wavelengths
Wavelengths λ (in vacuo) are specified in one line in ddscat.par consisting of values for 4 variables: WAVINI WAVEND NWAV CDIVID where WAVINI and WAVEND are real numbers, NWAV is an integer, and CDIVID is a character variable. • If CDIVID = ’LIN’, the λ will be uniformly spaced beween WAVINI and WAVEND. • If CDIVID = ’INV’, the λ will be uniformly spaced in 1/λ between WAVINI and WAVEND • If CDIVID = ’LOG’, the λ will be uniformly spaced in log(λ) between WAVINI and WAVEND • If CDIVID = ’TAB’, the λ will be read from a user-supplied file wave.tab, with one wavelength per line. For this case, the values of WAVINI, WAVEND, and NWAV will be disregarded. The sample ddscat.par file specifies that the calculations be done for a single wavelength (λ = 0.50). The units must be the same as the wavelength units used in the file specifying the refractive index. In this case, we are using µm.
9.10
Refractive Index of Ambient Medium, mmedium
In some cases the target of interest will be immersed in a transparent medium (e.g., water) with a refractive index different from vacuum. The refractive index mmedium should be specified here. As the medium is assumed to be transparent, mmedium is a real number. DDSCAT will calculate the scattering properties for the target immersed in the ambient medium. If the target is located in a vaccum, set mmedium = 1.
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