Starting a simulation

To start the simulation goto the directory containing the input files mcphas.ini, mcphas.j, etc. and type

mcphas

to run the program generating stepwise $H-T$ values in a loop given by mcphas.ini (you can also press the symbol in the McPhase - Explorer window). The complete syntax of the command is actually

mcphas[options] [file]

Adding the name of a file will run the program with an input file – file contains T(K) Hi(T) Hj(T) Hk(T) values to be calculated in columns 3, 5, 6, 7, respectively (ijk coordinates defined by $j\vert\vert b$, $k\vert\vert(a \times b)$ and $i$ normal to $k$ and $j$ ) if [file] is not given, xmin xmax xstep (xT xHa xHb xHc) ymin ymax ystep (yT yHa yHb yHc) is read from file mcphas.ini and phase diagram is calculated

Options are

-h

to print help and version of McPhas.

-stamax 14

end mcphas if standard deviation exceeds 14.

-a

avoid overwriting output files in results, append new results to existing files

-doeps

refine strain epsilon using elastic and magnetoelastic constants see also section 11 for how to enter these constants into mcphas.j

-cd

treat classical dipole interaction internally (for the the first three the interaction operators $I_1,I_2,I_3$, which must correspond to angular momentum $J$, such as in the so1ion module) and it is assumed that $(g_J.I_1,g_J.I_2,g_J.I_3)$ are the components of the magnetic moment ($g_J$ given in sipf file, thus setting it zero there will exclude the corresponding ion from the dipolar interaction calculation - this is the way for example to include phonon degrees of freedom in such a calculation), the classical dipole interaction is calculated using the series expansion for the classical dipolar expansion given by Bowden[30]. Using this option permits to avoid a large number of interaction constants in mcphas.j. For the mean field loop the dipolar interaction in equation (26) of[30] has to be evaluated for $q=0$, which is ambigous due to a factor $q_{\alpha}q_{\beta}/q^2$ which appears in the first term ”I” in equation (26) of[30]. This ambigousity reflects the shape of the sample. The factor $q_{\alpha}q_{\beta}/q^2$ is approximated in mcphas by zero, i.e. ”I=0” or $q_{\alpha}q_{\beta}/q^2 \sim 0$. This choice corresponds to setting $D_{zz}(q=0)=D^{eff}_{zz}(q=0)$ in [1], p. 236 - on this page the energy in applied magnetic field is given, which includes the demagnetising factor according to equation (5.5.6). Adding the energy of the demagnetizing field (see [] equation (6.49)) and introducing the internal applied magnetic field according to [1], p. 236, equation (5.5.9) it can be seen, that the energy depends on $D^{eff}_{zz}(q=0)$ and the internal magnetic field $H_I$. The magnetic structure can be calculated my mcphasit without taking into account explicitely the demagnetisation tensor. Note that the demagnetisation tensor (sample shape) will influence the excitations, see eplanation of option -cd for mcdisp for details.

-cel 0.0001

to compute elastic constants by applying stress of 0.0001 GPa. This needs more computation time, because one by one each component of the stress tensor is set to 0.0001 GPa and a mean field loop is done to compute the results strains. In this way, elastic constants can be obtained, which can be compared to experiment.

-cv 0.01

to compute specific heat by applying temperature step of 0.01K and computing derivative of energy u

-v

to enable verbose mode with lots of messages of McPhas. Specifically the verbose mode enables the following features:

• more information is printed out,
• the q-vectors file ./results/mcphas.qvc will contain the explicit spin configurations
• the display on screen (ghostview window using ./results/.sps.eps) will be updated not only when a H-T point has been finished but always when a structure with smaller free energy has been stabilised

-prefix 001

try to read files starting with 001, e.g. 001mcphas.ini, if these exist, otherwise take standard input files, check if in mcphas.ini there are parameters such as 001xmin and use those, if these are given AFTER the the standard parameters (i.e. xmin=0 ...001xmin=1 will set xmin to 1 whereas 001xmin=1 ...xmin=0 will set xmin to 0). Output goes to files results/001mcphas*.* (option ideal for parallel processes). Wildcard * may be used in the prefix option, e.g. -prefix 00* will run mcphas with all prefixes in the mcphas.ini file starting with 00. Note, some shells will need the syntax -prefix ”00*” for wildcards.

-read 001

try to read output files of previous calculation starting with 001, e.g. results/001mcphas.fum, if these exist, and if the x-y-T-Ha-Hb-Hc point is found and the calculation was stable (i.e. free energy in results/001mcphas.fum not zero do not recalculate it but take results from this previous calculation and store those. Option to recalculate nonstable points only.

-t 3

Takes structure number 3 in mcphas.tst and no other structures and traces this solution, i.e. no other structures are calculated and no comparison of free energies is done.

mcphasit

to start mcphase in commandline mode without opening any window

Exercises:

• Look at the input files for McPhase given in the directory examples/ndcu2b_new. How many atoms are contained in the crystallographic basis ?
• Start the simulation by typing the command mcphas.