Contents: 
    1) Calculation method (for mixtures)
    2) Important notice (for mixtures)
    3) Description of the input parameters


    1) Calculation method (for mixtures)
    ====================================

 <extracted from doc of f1f2_calc_mix>

	i) calculate the scattering factors f1_i and f2_i for each elements by
	   calling f1f2_calc
	ii) Calculate the atomic composition f_i of each element in the 
	   mixture by calling parse_mixture 
	iii) The mean scattering factors for the mixture ate the 
	   averaged scattering factors from the elements (f_i):
		f1 = Sum{ f_i * f1_i }  (similarly for f2)
	iv) Returns the value in the correspondent units:
	
	The MW (molecular weight) is MW = Sum { f_i * Ai), being Ai
		the atomic weight of the i-th element available in 
		the molecule (mixture) formula.


    2) Important notice (for mixtures)
    ==================================


 <extracted from doc of f1f2_calc_mix>
	   A mixture can be defined, either 
		i) as a function of the weight fractions 
		ii) as a molecular mixture. 

	   Therefore, water can be either defined as "H2O" (molecule) 
		or "H(0.11)O(0.89)" (weight). 	
	
		FOR f1f2_calc_mix CALCULATIONS, THE MIXTURE SHOULD BE
		DEFINED IN ITS MOLECULAR FORM (like "H2O") AND NOT IN ITS 
		WEIGHT FRACTION (like "H(0.11)O(0.89)"). 

		The reason is that the averaging coefficient f_i is
		is the atomic fraction and NOT the weight fraction. 
		
		When defining the mixture using weight fractions,
		XOP calculates the atomic fraction from the weight fractions
		(done in parse_mixture) but the result is inaccurate when
		the molecular formula is unknown, because the molecular
		weight is unknown.

		PLEASE NOTE THAT THE PRE-DEFINED COMPOUNDS (IN DABAX
		FILE Compounds.dat CONTAIN THE MIXTURE FORMULA, AND IN
		SOME CASES (THE COMPOUNDS OBTAINED FROM NIST DATABASE)
		THE MOLECULAR FORMULA IS NOT PRESENT (ONLY WEIGHT FORMULA)
		THEREFORE THE SCATTERING FACTORS AND RELATED VALUES MAY BE 
		WRONG.

		Practical tip: 

		For cross section, scattering factors and reflectivity, use
		    xf1f2 and try to define your material in molecular
		    form. 
		For calculating attenuation coefficients, use xCrossSec 
		    (and not xf1f2) because:
		    i) it is more accurate in case of defining the material
			by its weight fractions.
		    ii) it used all channels in the attenuation (contrary to 
		        xf1f2 that only uses photoelectron cross section). 



    3) Description of the input parameters
    ======================================
This is a description of the input parameters for xf1f2

datasets: 
	Select a DABAX file from where the f1f2 scattering factors
	will be read. The "all" option is also available, and 
	in this case all datasets are used (useful for comparisons).

material: Select the desired material type. Three options are available:
	a) Element(formula) calculation for a single element. 
	b) Mixture(formula) calculation for a compound or a mixture. The user 
		defines the mixture formula. 
		The syntaxis is the following: string with the chemical
		formula of the compound(s) with their weight fraction between 
		parenthesis.
		Examples:
		H20
		SiO2(0.807)B2O3(0.129)Na2O(0.038)Al2O3(0.022)K2O(0.004)
		H2O(0.99)Cl2(0.01)
		H(0.11189834)O(0.88810166)
	c) Mixture(table) calculation for a compound or a mixture defined
		in the included mixture-table (from dabax file Compounds.dat)

table: Compound table, only available if the c) option in material is chosen.

formula: The element or compound or mixture formula, depending the "material"
		selection.

density: The mixture density in the case that option b) has been
		selected.

calculate: select the calculation magnitude. The "all" option is also 
	available, and in this case all datasets are used (useful for 
	comparisons). The "all" option here is incompatible with the 
	"all" option in the "datasets" entry.


Energy [eV] grid: the abscissas values for the calculations. Two 
		options are available: 
		a) Standard, 100 points in the [0,3] interval.
		b) User-defined, with the parameters described below.
		c) Single-value, with the energy to be entered in 
		   "Starting Energy [eV]" box.

Stating Energy [eV]: the initial energy value (only available if either
		b) or c) is chosen in the "Energy [eV] grid" entry.

To: the last energy value (only available if b) is chosen in the
		"Energy [eV] grid" entry.

Number of points: the number of energy points (only available if b) is 
		chosen in the "sin_theta/lambda grid" entry.

Grazing angle: the surface grazing angle (only available when 
		"reflectivity" calculations are set in "Calculate" entry.
		Two options are available: 
		a) Single-value, for calculations of Reflectivity versus
		   energy.
		b) User-defined, for calculations of Reflectivity versus
		   angle, or reflectivity versus both angle and energy 
		   (2D plot).

Roughness rms [A]: The surface RMS roughness (only available when
		"reflectivity" calculations are set in "Calculate" entry.

Stating Graz angle [mrad]: the grazing angle (when single-value is set) or
		the initial grazing angle value (only available for
		reflectivity calculations).

To: the last grazing angle value (only available if both b) is chosen in the
		"Grazing angle" entry and for reflectivity calculations).

Number of angular points: the number of angle points (only available if both 
		b) is chosen in the "Grazing angle" entry and for reflectivity 
		calculations).

