1 | # core_shell_parallelepiped model |
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2 | # Note: model title and parameter table are inserted automatically |
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3 | r""" |
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4 | Calculates the form factor for a rectangular solid with a core-shell structure. |
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5 | **The thickness and the scattering length density of the shell or "rim" |
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6 | can be different on all three (pairs) of faces.** |
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7 | |
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8 | The form factor is normalized by the particle volume $V$ such that |
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9 | |
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10 | .. math:: |
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11 | |
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12 | I(q) = \text{scale}\frac{\langle f^2 \rangle}{V} + \text{background} |
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13 | |
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14 | where $\langle \ldots \rangle$ is an average over all possible orientations |
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15 | of the rectangular solid. |
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16 | |
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17 | An instrument resolution smeared version of the model is also provided. |
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18 | |
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19 | |
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20 | Definition |
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21 | ---------- |
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22 | |
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23 | The function calculated is the form factor of the rectangular solid below. |
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24 | The core of the solid is defined by the dimensions $A$, $B$, $C$ such that |
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25 | $A < B < C$. |
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26 | |
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27 | .. image:: img/core_shell_parallelepiped_geometry.jpg |
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28 | |
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29 | There are rectangular "slabs" of thickness $t_A$ that add to the $A$ dimension |
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30 | (on the $BC$ faces). There are similar slabs on the $AC$ $(=t_B)$ and $AB$ |
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31 | $(=t_C)$ faces. The projection in the $AB$ plane is then |
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32 | |
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33 | .. image:: img/core_shell_parallelepiped_projection.jpg |
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34 | |
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35 | The volume of the solid is |
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36 | |
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37 | .. math:: |
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38 | |
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39 | V = ABC + 2t_ABC + 2t_BAC + 2t_CAB |
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40 | |
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41 | **meaning that there are "gaps" at the corners of the solid.** |
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42 | |
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43 | The intensity calculated follows the :ref:`parallelepiped` model, with the core-shell |
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44 | intensity being calculated as the square of the sum of the amplitudes of the |
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45 | core and shell, in the same manner as a core-shell model. |
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46 | |
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47 | **For the calculation of the form factor to be valid, the sides of the solid |
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48 | MUST be chosen such that** $A < B < C$. |
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49 | **If this inequality is not satisfied, the model will not report an error, |
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50 | and the calculation will not be correct.** |
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51 | |
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52 | FITTING NOTES |
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53 | If the scale is set equal to the particle volume fraction, |phi|, the returned |
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54 | value is the scattered intensity per unit volume, $I(q) = \phi P(q)$. |
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55 | However, **no interparticle interference effects are included in this calculation.** |
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56 | |
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57 | There are many parameters in this model. Hold as many fixed as possible with |
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58 | known values, or you will certainly end up at a solution that is unphysical. |
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59 | |
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60 | Constraints must be applied during fitting to ensure that the inequality |
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61 | $A < B < C$ is not violated. The calculation will not report an error, |
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62 | but the results will not be correct. |
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63 | |
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64 | The returned value is in units of |cm^-1|, on absolute scale. |
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65 | |
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66 | NB: The 2nd virial coefficient of the core_shell_parallelepiped is calculated |
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67 | based on the the averaged effective radius $(=\sqrt{(A+2t_A)(B+2t_B)/\pi})$ |
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68 | and length $(C+2t_C)$ values, and used as the effective radius |
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69 | for $S(Q)$ when $P(Q) * S(Q)$ is applied. |
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70 | |
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71 | .. Comment by Miguel Gonzalez: |
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72 | The later seems to contradict the previous statement that interparticle interference |
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73 | effects are not included. |
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74 | |
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75 | To provide easy access to the orientation of the parallelepiped, we define the |
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76 | axis of the cylinder using three angles $\theta$, $\phi$ and $\Psi$. |
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77 | (see :ref:`cylinder orientation <cylinder-angle-definition>`). |
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78 | The angle $\Psi$ is the rotational angle around the *long_c* axis against the |
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79 | $q$ plane. For example, $\Psi = 0$ when the *short_b* axis is parallel to the |
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80 | *x*-axis of the detector. |
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81 | |
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82 | .. figure:: img/parallelepiped_angle_definition.jpg |
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83 | |
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84 | Definition of the angles for oriented core-shell parallelepipeds. |
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85 | |
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86 | .. figure:: img/parallelepiped_angle_projection.jpg |
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87 | |
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88 | Examples of the angles for oriented core-shell parallelepipeds against the |
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89 | detector plane. |
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90 | |
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91 | Validation |
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92 | ---------- |
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93 | |
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94 | The model uses the form factor calculations implemented in a c-library provided |
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95 | by the NIST Center for Neutron Research (Kline, 2006). |
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96 | |
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97 | References |
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98 | ---------- |
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99 | |
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100 | P Mittelbach and G Porod, *Acta Physica Austriaca*, 14 (1961) 185-211 |
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101 | Equations (1), (13-14). (in German) |
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102 | |
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103 | """ |
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104 | |
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105 | import numpy as np |
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106 | from numpy import pi, inf, sqrt |
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107 | |
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108 | name = "core_shell_parallelepiped" |
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109 | title = "Rectangular solid with a core-shell structure." |
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110 | description = """ |
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111 | P(q)= |
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112 | """ |
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113 | category = "shape:parallelepiped" |
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114 | |
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115 | # ["name", "units", default, [lower, upper], "type","description"], |
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116 | parameters = [["sld_core", "1e-6/Ang^2", 1, [-inf, inf], "sld", |
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117 | "Parallelepiped core scattering length density"], |
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118 | ["sld_a", "1e-6/Ang^2", 2, [-inf, inf], "sld", |
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119 | "Parallelepiped A rim scattering length density"], |
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120 | ["sld_b", "1e-6/Ang^2", 4, [-inf, inf], "sld", |
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121 | "Parallelepiped B rim scattering length density"], |
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122 | ["sld_c", "1e-6/Ang^2", 2, [-inf, inf], "sld", |
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123 | "Parallelepiped C rim scattering length density"], |
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124 | ["sld_solvent", "1e-6/Ang^2", 6, [-inf, inf], "sld", |
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125 | "Solvent scattering length density"], |
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126 | ["length_a", "Ang", 35, [0, inf], "volume", |
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127 | "Shorter side of the parallelepiped"], |
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128 | ["length_b", "Ang", 75, [0, inf], "volume", |
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129 | "Second side of the parallelepiped"], |
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130 | ["length_c", "Ang", 400, [0, inf], "volume", |
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131 | "Larger side of the parallelepiped"], |
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132 | ["thick_rim_a", "Ang", 10, [0, inf], "volume", |
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133 | "Thickness of A rim"], |
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134 | ["thick_rim_b", "Ang", 10, [0, inf], "volume", |
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135 | "Thickness of B rim"], |
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136 | ["thick_rim_c", "Ang", 10, [0, inf], "volume", |
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137 | "Thickness of C rim"], |
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138 | ["theta", "degrees", 0, [-inf, inf], "orientation", |
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139 | "In plane angle"], |
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140 | ["phi", "degrees", 0, [-inf, inf], "orientation", |
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141 | "Out of plane angle"], |
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142 | ["psi", "degrees", 0, [-inf, inf], "orientation", |
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143 | "Rotation angle around its own c axis against q plane"], |
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144 | ] |
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145 | |
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146 | source = ["lib/gauss76.c", "core_shell_parallelepiped.c"] |
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147 | |
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148 | |
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149 | def ER(length_a, length_b, length_c, thick_rim_a, thick_rim_b, thick_rim_c): |
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150 | """ |
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151 | Return equivalent radius (ER) |
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152 | """ |
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153 | |
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154 | # surface average radius (rough approximation) |
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155 | surf_rad = sqrt((length_a + 2.0*thick_rim_a) * (length_b + 2.0*thick_rim_b) / pi) |
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156 | |
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157 | height = length_c + 2.0*thick_rim_c |
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158 | |
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159 | ddd = 0.75 * surf_rad * (2 * surf_rad * height + (height + surf_rad) * (height + pi * surf_rad)) |
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160 | return 0.5 * (ddd) ** (1. / 3.) |
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161 | |
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162 | # VR defaults to 1.0 |
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163 | |
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164 | # parameters for demo |
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165 | demo = dict(scale=1, background=0.0, |
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166 | sld_core=1, sld_a=2, sld_b=4, sld_c=2, sld_solvent=6, |
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167 | length_a=35, length_b=75, length_c=400, |
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168 | thick_rim_a=10, thick_rim_b=10, thick_rim_c=10, |
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169 | theta=0, phi=0, psi=0, |
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170 | length_a_pd=0.1, length_a_pd_n=1, |
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171 | length_b_pd=0.1, length_b_pd_n=1, |
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172 | length_c_pd=0.1, length_c_pd_n=1, |
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173 | thick_rim_a_pd=0.1, thick_rim_a_pd_n=1, |
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174 | thick_rim_b_pd=0.1, thick_rim_b_pd_n=1, |
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175 | thick_rim_c_pd=0.1, thick_rim_c_pd_n=1, |
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176 | theta_pd=10, theta_pd_n=1, |
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177 | phi_pd=10, phi_pd_n=1, |
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178 | psi_pd=10, psi_pd_n=1) |
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179 | |
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180 | qx, qy = 0.2 * np.cos(2.5), 0.2 * np.sin(2.5) |
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181 | tests = [[{}, 0.2, 0.533149288477], |
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182 | [{}, [0.2], [0.533149288477]], |
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183 | [{'theta':10.0, 'phi':10.0}, (qx, qy), 0.032102135569], |
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184 | [{'theta':10.0, 'phi':10.0}, [(qx, qy)], [0.032102135569]], |
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185 | ] |
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186 | del qx, qy # not necessary to delete, but cleaner |
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