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diff --git a/fg21sim/extragalactic/clusters/units.py b/fg21sim/extragalactic/clusters/units.py
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+# Copyright (c) 2017 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Commonly used units and their conversions relations, as well as constants.
+
+Astropy's units system is very powerful, but also very slow,
+and may even be the speed bottleneck of the program.
+
+This module provides commonly used units conversions by holding
+them directly in a class, thus avoid repeated/unnecessary calculations.
+"""
+
+import astropy.units as au
+import astropy.constants as ac
+
+
+class Units:
+    """
+    Commonly used units, especially in the CGS unit system.
+    """
+    # Unit for electron momentum (p), thus its value is the Lorentz factor
+    mec = ac.m_e.cgs.value*ac.c.cgs.value  # [g cm / s]
+
+
+class UnitConversions:
+    """
+    Commonly used units conversion relations.
+
+    Hold the conversion relations directly to avoid repeated/unnecessary
+    calculations.
+    """
+    Msun2g = au.solMass.to(au.g)
+    g2Msun = au.g.to(au.solMass)
+    Gyr2s = au.Gyr.to(au.s)
+    s2Gyr = au.s.to(au.Gyr)
+    kpc2cm = au.kpc.to(au.cm)
+    cm2kpc = au.cm.to(au.kpc)
+    Mpc2cm = au.Mpc.to(au.cm)
+    cm2Mpc = au.cm.to(au.Mpc)
+    Mpc2km = au.Mpc.to(au.km)
+    km2Mpc = au.km.to(au.Mpc)
+    kpc2km = au.kpc.to(au.km)
+    km2kpc = au.km.to(au.kpc)
+    km2cm = au.km.to(au.cm)
+    keV2erg = au.keV.to(au.erg)
+
+
+class Constants:
+    """
+    Commonly used constants, especially in the CGS unit system.
+
+    Astropy's constants are stored in SI units by default.
+    When request a constant in CGS unit system, additional (and slow)
+    conversions required.
+    """
+    # Speed of light
+    c = ac.c.cgs.value  # [cm/s]
+    # Atomic mass unit (i.e., a.m.u.)
+    u = ac.u.cgs.value  # [g]
+    # Gravitational constant
+    G = ac.G.cgs.value  # [cm^3/g/s^2]
+
+    # Mean molecular weight
+    # Ref.: Ettori et al, 2013, Space Science Review, 177, 119-154, Eq.(6)
+    mu = 0.6