diff --git a/src/physics/units.cpp b/src/physics/units.cpp
index b9acaf9..ef6b633 100644
--- a/src/physics/units.cpp
+++ b/src/physics/units.cpp
@@ -1,184 +1,191 @@
 /*-------------------------------------------------------------------------------
 
 Copyright (c) 2015 F. Georget <fabieng@princeton.edu>, Princeton University
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without modification,
 are permitted provided that the following conditions are met:
 
 1. Redistributions of source code must retain the above copyright notice, this
 list of conditions and the following disclaimer.
 
 2. Redistributions in binary form must reproduce the above copyright notice,
 this list of conditions and the following disclaimer in the documentation and/or
 other materials provided with the distribution.
 
 3. Neither the name of the copyright holder nor the names of its contributors
 may be used to endorse or promote products derived from this software without
 specific prior written permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
 ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
 ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 -----------------------------------------------------------------------------*/
 
 #include "units.hpp"
 
 #include <algorithm>
 
 namespace specmicp {
 namespace units {
 
 scalar_t convert_pressure(scalar_t pressure_si, LengthUnit length_unit)
 {
     scalar_t pressure;
     switch (length_unit) {
     case LengthUnit::meter:
         pressure = pressure_si;
         break;
     case LengthUnit::decimeter:
         pressure = 1e-2*pressure_si;
         break;
     case LengthUnit::centimeter:
         pressure = 1e-4*pressure_si;
     }
     return pressure;
 }
 
+AmountUnit parse_amount_unit(const std::string& unit)
+{
+    AmountUnit amounts;
+    parse_amount_unit(unit, amounts);
+    return amounts;
+}
+
 void parse_amount_unit(std::string unit, AmountUnit& amounts)
 {
 
     auto n = unit.find("/");
     amounts.type = AmountUnitType::Unknown;
     amounts.factor_si = -1;
     scalar_t factor;
     if (n == std::string::npos)
     {
         unit.erase(std::remove(unit.begin(), unit.end(), ' '),
                        unit.end());
         factor = is_mass_unit(unit);
         if (factor > 0)
         {
             amounts.type = AmountUnitType::Mass;
             amounts.factor_si = factor;
             return;
         }
         factor = is_mole_unit(unit);
         if (factor > 0)
         {
             amounts.type = AmountUnitType::NumberOfMoles;
             amounts.factor_si = factor;
             return;
         }
         factor = is_volume_unit(unit);
         if (factor > 0)
         {
             amounts.type = AmountUnitType::Volume;
             amounts.factor_si = factor;
             return;
         }
     }
     else
     {
         // composite unit
         std::string numer = unit.substr(0, n);
         numer.erase(std::remove(numer.begin(), numer.end(), ' '),
                        numer.end());
         std::string denom = unit.substr(n+1, unit.size()-n);
         denom.erase(std::remove(denom.begin(), denom.end(), ' '),
                        denom.end());
         scalar_t factor_numer = -1;
         scalar_t factor_denom = -1;
 
         factor_denom = is_volume_unit(denom);
         if (factor_denom > 0)
         {
             // concentration
             factor_numer = is_mass_unit(numer);
             if (factor_numer > 0)
             {
                 amounts.type = AmountUnitType::MassConcentration;
                 amounts.factor_si = factor_numer / factor_denom;
                 return;
             }
             factor_numer = is_mole_unit(numer);
             if (factor_numer > 0)
             {
                 amounts.type = AmountUnitType::MoleConcentration;
                 amounts.factor_si = factor_numer / factor_denom;
                 return;
             }
             return;
         }
         factor_denom = is_mass_unit(denom);
         if (factor_denom > 0)
         {
             factor_numer = is_mole_unit(numer);
             if (factor_numer > 0)
             {
                 amounts.type = AmountUnitType::Molality;
                 amounts.factor_si = factor_numer / factor_denom;
                 return;
             }
         }
     }
 }
 
 scalar_t is_mass_unit(const std::string& unit)
 {
     scalar_t factor = -1;
     if (unit == "kg" or unit == "kilogram")
         factor = 1;
     else if (unit == "g" or unit == "gram")
         factor = 1e-3;
     return factor;
 }
 
 scalar_t is_length_unit(const std::string& unit)
 {
     scalar_t factor = -1;
     if (unit == "m" or unit == "meter")
         factor = 1;
     else if (unit == "dm" or unit == "decimeter")
         factor = 0.1;
     else if (unit == "cm" or unit == "centimeter")
         factor = 0.01;
     else if (unit == "mm" or unit == "millimeter")
         factor = 1e-3;
     return factor;
 }
 
 scalar_t is_volume_unit(const std::string& unit)
 {
     scalar_t factor = -1;
     if (unit == "m^3" or unit == "cubicmeter")
         factor = 1;
     else if (unit == "dm^3" or unit == "cubicdecimeter")
         factor = 1e-3;
     else if (unit == "cm^3" or unit == "cubiccentimeter")
         factor = 1e-6;
     else if (unit == "mm^3" or unit == "cubicmillimeter")
         factor = 1e-9;
     return factor;
 }
 
 scalar_t is_mole_unit(const std::string &unit)
 {
     scalar_t factor = -1;
     if (unit == "mol" or unit == "moles")
         factor = 1;
     else if (unit == "mmol" or unit == "millimoles")
         factor = 1e-3;
     return factor;
 }
 
 } //end namespace units
 } //end namespace specmicp
diff --git a/src/physics/units.hpp b/src/physics/units.hpp
index a2bde3a..2406ec0 100644
--- a/src/physics/units.hpp
+++ b/src/physics/units.hpp
@@ -1,172 +1,184 @@
 /*-------------------------------------------------------------------------------
 
 Copyright (c) 2014,2015 F. Georget <fabieng@princeton.edu>, Princeton University
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without modification,
 are permitted provided that the following conditions are met:
 
 1. Redistributions of source code must retain the above copyright notice, this
 list of conditions and the following disclaimer.
 
 2. Redistributions in binary form must reproduce the above copyright notice,
 this list of conditions and the following disclaimer in the documentation and/or
 other materials provided with the distribution.
 
 3. Neither the name of the copyright holder nor the names of its contributors
 may be used to endorse or promote products derived from this software without
 specific prior written permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
 ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
 ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 -----------------------------------------------------------------------------*/
 
 #ifndef SPECMICP_UNITS_UNITS
 #define SPECMICP_UNITS_UNITS
 
 #include "../types.hpp"
 
 //! \file physics/units.hpp
 //! \brief units management and conversion
 
 namespace specmicp {
 
 //! \namespace specmicp::units
 //! \brief Units management
 namespace units {
 
 //! units used for length
 enum class LengthUnit {
     meter,
     decimeter,
     centimeter
 };
 
 //! units used for mass
 enum class MassUnit {
     kilogram,
     gram
 };
 
 
 //! \brief Simple struct which contains the unit information
 struct UnitsSet
 {
     MassUnit mass; //!< The unit to use for mass
     LengthUnit length; //!< The unit to use for length (also area and volume)
 
     //! \brief Defaults are the SI unit system
     UnitsSet():
         mass(MassUnit::kilogram),
         length(LengthUnit::meter)
     {}
 };
 
 //! \brief Base class that handles the units
 //!
 //!  To be inherited by other classes that need units
 class UnitBaseClass
 {
 public:
     UnitBaseClass() {}
     UnitBaseClass(UnitsSet units):
         m_units(units)
     {}
     //! \brief Return the units
-    UnitsSet get_units() {return m_units;}
+    const UnitsSet& get_units() const {return m_units;}
 
     //! \brief Set the units
     void set_units(UnitsSet units) {m_units = units;}
 
     //! \brief Return the Mass unit
     MassUnit mass_unit() const {return m_units.mass;}
     //! \brief Return the Length unit
     LengthUnit length_unit() const {return m_units.length;}
 
     //! \brief Return the mass unit
     MassUnit& mass_unit() {return m_units.mass;}
     //! \brief Return the length unit
     LengthUnit& length_unit() {return m_units.length;}
 
 private:
     UnitsSet m_units;
 };
 
 
 //! \brief the type of an 'amount' unit
 enum class AmountUnitType
 {
     Mass,
     Volume,
     NumberOfMoles,
     MoleConcentration,
     MassConcentration,
     Molality,
     Unknown
 };
 
 //! \brief Description of an amount unit
 struct AmountUnit
 {
     AmountUnitType type; //!< The unit type
     scalar_t factor_si; //!< unit*factor_si = SI unit
 };
 
 //! \brief Find the unit and factor of 'unit'
 //!
 //! \param unit the unit to analyse
 //! \param amounts
 void parse_amount_unit(std::string unit, AmountUnit& amounts);
 
+//! \brief Finf the unit and factor of 'unit'
+AmountUnit parse_amount_unit(const std::string& unit);
+
 //! \brief Check if 'unit' is a mass unit
 //!
 //! \return if it is a mass unit return the factor to transform it to its SI equivalent, else return a negative number
 scalar_t is_mass_unit(const std::string& unit);
 //! \brief Check if 'unit' is a length unit
 //!
 //! \return if it is a length unit return the factor to transform it to its SI equivalent, else return a negative number
 scalar_t is_length_unit(const std::string& unit);
 //! \brief Check if 'unit' is a volume unit
 //!
 //! \return if it is a volume unit return the factor to transform it to its SI equivalent, else return a negative number
 scalar_t is_volume_unit(const std::string& unit);
 //! \brief Check if 'unit' is a mole unit
 //!
 //! \return if it is a mole unit return the factor to transform it to its SI equivalent, else return a negative number
 scalar_t is_mole_unit(const std::string& unit);
 
 // Temperature
 // -----------
 
-//! Convert a temperature from Celsius to Kelvin
+//! \brief Convert a temperature from Celsius to Kelvin
 inline constexpr scalar_t celsius(scalar_t tc) {return tc + 273.15;}
-//! Convert a temperature from Kelvin to Celsius
+//! \brief Convert a temperature from Kelvin to Celsius
 inline constexpr scalar_t to_celsius(scalar_t tk) { return tk - 273.15;}
 
+//! \brief Convert a pressure from bar to pascal
+inline constexpr scalar_t bar(scalar_t pb) {return 1e5*pb;}
+//! \brief Convert a pressure from pascal to bar
+inline constexpr scalar_t to_bar(scalar_t ppa) {return 1e-5*ppa;}
+//! \brief Convert a pressure from atm to pascal
+inline constexpr scalar_t atm(scalar_t patm) {return 101325*patm;}
+//! \brief Convert a pressure from pascal to atm
+inline constexpr scalar_t to_atm(scalar_t ppa) {return ppa/101325;}
+
 // Volume
 // ------
 
 //! Convert liter to cubic meter
 inline constexpr scalar_t liter(scalar_t vl) {return 1e-3*vl;}
 //! convert cubic meter to liter
 inline constexpr scalar_t to_liter(scalar_t vcm) {return 1e3*vcm;}
 
 // Pressure
 // --------
 
 
 scalar_t SPECMICP_DLL_PUBLIC convert_pressure(scalar_t pressure_si, LengthUnit length_unit);
 
 } // end namespace units
 } // end namespace specmicp
 
 #endif // SPECMICP_UNITS_UNITS