nnm:faq
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nnm:faq [2022/01/11 19:43] carola.burkl [Licensing] |
nnm:faq [2022/01/13 15:08] carola.burkl [What is the difference between "List view" and "Tree view"?] |
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| - | ==== How can I speed up my calculations with respect to CPU time? ==== | ||
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| - | The most obvious way is to reduce the **number of grid points** you are using. | ||
| - | For instance, for the following pn junction simulation, a grid spacing of **1 nm** was used (gray lines). | ||
| - | If one is using a coarse grid of only **10 nm**, the calculated values (squares) agree very well with the calculated values of the thin lines. | ||
| - | <figure pn_grid_1> | ||
| - | {{ :nnm:pn_junction_coarse_grid.jpg?direct&600 |}} | ||
| - | <caption> Conduction and (heavy and light hole) valence band edges and Fermi level for a 300 nm long pn junction. | ||
| - | </caption> | ||
| - | </figure> | ||
| - | <figure pn_grid_2> | ||
| - | {{ :nnm:pn_carrier_concentration.jpg?direct |}} | ||
| - | {{ :nnm:pn_junction_coarse_grid_densities.jpg?direct&600 |}} | ||
| - | <caption> Hole (blue) and electron (red) densities of the pn junction in units of $10^{18} {\rm ~cm^{-3}}$. The gray lines are from simulations using a 1 nm grid spacing. The squares are from a simulation that uses only a 10 nm grid resolution. Note that the center coordinate of this plot is x=160 nm. The depletion width for the holes is around $w_{\rm p}\approx 50 {\rm ~nm}$, for the electrons it is $w_{\rm n}\approx 10 {\rm ~nm}$ which is of the order of the grid spacing. Even in this case, the calculated electron density is reasonably accurate. | ||
| - | </caption> | ||
| - | </figure> | ||
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| - | The difference in CPU time comes from the fact that for the 10 nm resolution the dimension of the matrix that is used for discretizing the Poisson equation is 30, while in the case for the 1 nm grid spacing it has the dimension 300. | ||
| - | The proper choice of an optimal grid spacing is very relevant for 2D and 3D simulations, as can be seen in the following. | ||
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| - | 1D simulation (length of sample: x = 300 nm) | ||
| - | * 1 nm grid spacing: dimension of Poisson matrix: $N=300$ | ||
| - | * 10 nm grid spacing: dimension of Poisson matrix: $N=30$ | ||
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| - | 2D simulation (length of sample: x = 300 nm, y = 300 nm) | ||
| - | * 1 nm grid spacing: dimension of Poisson matrix: $N=300 \cdot 300 = 90,000$ | ||
| - | * 10 nm grid spacing: dimension of Poisson matrix: $N=30 \cdot 30 = 900$ | ||
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| - | 3D simulation (length of sample: x = 300 nm, y = 300 nm, z = 300 nm) | ||
| - | * 1 nm grid spacing: dimension of Poisson matrix: $N=300 \cdot 300 \cdot 300 = 27,000,000$ | ||
| - | * 10 nm grid spacing: dimension of Poisson matrix: $N=30 \cdot 30 \cdot 30 = 27,000$ | ||
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| - | If a quantum mechanical simulation is performed, the numerical effort of eigenvalue solvers increases with the number of grid points $N$ with order $O\left(N^2\right)$. | ||
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| - | ---- | ||
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| - | ==== What is the difference between "List view" and "Tree view"? ==== | ||
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| - | You can switch between these options using: | ||
| - | <code> | ||
| - | Tools ==> Options ==> View ==> Output folder browser | ||
| - | </code> | ||
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| - | ''List view'' is the default. | ||
| - | ''Tree view'' is convenient if you do a voltage sweep with nextnano++ as for each voltage, one has a different output folder labeled ''sweep_001/'', or if your simulation output folders are randomly distributed over your hard disk. | ||
| - | The following screenshots highlight the differences. | ||
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| - | ''List view'' (default) | ||
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| - | {{ :nnm:view_listview.jpg?direct |}} | ||
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| - | ''Tree view'' | ||
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| - | {{ :nnm:view_treeview.jpg?direct |}} | ||
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| ==== Can I add new materials to the database? ==== | ==== Can I add new materials to the database? ==== | ||
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