Units and constants¶
The units of MDAnalysis trajectories are the Å (ångström) for length and ps (picosecond) for time. Regardless of how the original MD format stored the trajectory data, MDAnalysis converts it to MDAnalysis units when reading the data in, and converts back when writing the data out. Analysis classes generally also use these default units. Exceptions to the default units are always noted in the documentation; for example, mass densities can be given in \(g/cm^3\).
Other base units are listed in the table Base units in MDAnalysis.
Quantity |
Unit |
SI units |
|---|---|---|
length |
Å |
\(10^{-10}\) m |
time |
ps |
\(10^{-12}\) s |
energy |
kJ/mol |
\(1.66053892103219 \times 10^{-21}\) J |
charge |
\(e\) |
\(1.602176565 \times 10^{-19}\) As |
force |
kJ/(mol·Å) |
\(1.66053892103219 \times 10^{-11}\) J/m |
speed |
Å/ps |
\(100\) m/s |
mass |
u |
\(1.66053906660(50) \times 10^{-27}\) kg |
angle |
degrees |
\(\frac{\pi}{180}\) rad |
Unit conversion¶
Quantities can be converted from units with convert().
convert() simply multiplies the initial quantity with a
precomputed conversion factor, as obtained from
get_conversion_factor().
The computed conversion factors for each quantity type are stored in MDAnalysis.units and shown below.
Constants¶
Length¶
Unit
Conversion factor
A
1
Angstrom
1
angstrom
1
femtometer
100000
fm
100000
nanometer
0.1
nm
0.1
picometer
100
pm
100
Å
1
Density¶
Unit
Conversion factor
A^{-3}
1
Angstrom^{-3}
1
Molar
1660.54
SPC
30.3718
TIP3P
29.8566
TIP4P
29.8864
nanometer^{-3}
1000
nm^{-3}
1000
water
30.0063
Å^{-3}
1
Time¶
Unit
Conversion factor
AKMA
20.4548
femtosecond
1000
fs
1000
nanosecond
0.001
ns
0.001
picosecond
1
ps
1
s
1e-12
sec
1e-12
second
1e-12
Charge¶
Unit
Conversion factor
Amber
18.2223
As
1.60218e-19
C
1.60218e-19
e
1
Speed¶
Unit
Conversion factor
A/ps
1
Angstrom/AKMA
0.0488882
Angstrom/femtosecond
1000
Angstrom/fs
1000
Angstrom/picosecond
1
Angstrom/ps
1
angstrom/femtosecond
1000
angstrom/fs
1000
angstrom/picosecond
1
m/s
100
nanometer/picosecond
0.1
nanometer/ps
0.1
nm/ns
100
nm/ps
0.1
pm/ps
100
Å/ps
1
Force¶
Unit
Conversion factor
J/m
1.66054e-11
N
1.66054e-11
Newton
1.66054e-11
kJ/(mol*A)
1
kJ/(mol*Angstrom)
1
kJ/(mol*nm)
10
kJ/(mol*Å)
1
kcal/(mol*Angstrom)
0.239006
Energy¶
Unit
Conversion factor
J
1.66054e-21
eV
0.0103643
kJ/mol
1
kcal/mol
0.239006