quantum’mechanics’for’bio2 materials’cmt.dur.ac.uk/sjc/biomaths/lecture1.pdf · aims’...
TRANSCRIPT
Quantum Mechanics for Bio-‐materials
M.Sc. Course in BioMathema7cs
Prof. Stewart Clark Department of Physics
Office 145 [email protected]
Aims
To provide the students with a working knowledge of modeling biomaterials at the atomic level using first principles electronic structure techniques. The course will be a mixture of lectures on the theory and methods of modeling materials from an electronic structure point of view leading to prac7cal, computa7onal sessions where the techniques will be put into prac7ce.
Contents
• (Very brief!) Introduc7on to quantum mechanics
• The many-‐par7cle (electron) problem – How we solve it – Density func7onal theory – Bloch’s Theorem
– Basis Sets – Varia7onal Method
• Prac7cal examples applied (computa7onal) applied to bio-‐molecules
Background Reading
• If you are not familiar with quantum mechanics, then you’ll need some background reading
• Please get any book from the library with a 7tle like “Introduc7on to quantum mechanics” form the library
• Read the chapters on understanding and solving the Schrodinger equa7on for simple cases
Some assump7ons about you
• You’ve limited experience about the physics of quantum mechanics
• You can do mathema7cs! • You have some interest an understanding about basic molecular biology (e.g. you know what amino acids are)
• You have a basic knowledge of chemistry (molecules are made of atoms, atoms are made of protons, neutrons and electrons)
• You know that electrons are important: they determine the proper7es of ma[er
Quantum Mechanics
• What is quantum mechanics?
The origins of quantum mechanics
• At the end of the 19th century it was thought that physics was more or less “solved”
• There were just a few minor issues to clear up
• Here’s one – the solar and atomic spectra:
“Black body radia7on” compared to the solar spectrum
A significant number of gaps in the spectrum which cannot be explained using classical physics
Apparent Pa[ern
• There’s a regularity to the spectrum • The wave-‐numbers (1/wavelength or number of waves per metre)
• This is an experimental observa7on
€
k ∝ 1na2 −
1nb2
⎛
⎝ ⎜
⎞
⎠ ⎟
Bohr Model of the Atom
• Nucleus in the centre surrounded by electrons in “orbits”
Analysis of Model
• Electrosta7c (Coulomb) force
• balances centripetal (rota7onal) force
• i.e. the system is in equilibrium when
€
FC =14πε 0
Ze2
r2
€
FR =mv 2
r
€
FC = FR
Quan7sa7on
• Bohr’s postulate of the atom takes the form • “The angular momentum of the electrons is quan7sed”
• This means that the angular momentum, L, can only take discrete mul7ples of a fundamental quan7ty
• where n is an integer and ħ is Plank’s constant
€
L = n = mvr
Solve for v and r
• We have
• giving
€
14πε 0
Ze2
r2=mv 2
rmvr = n
€
v =Ze2
4πε 0n
r =4πε 0
2n2
Zme2
Examine energy
• Kine7c energy:
• Poten7al Energy:
€
T =12mv 2 =
m22
Ze2
4πε 0
⎛
⎝ ⎜
⎞
⎠ ⎟
21n2
€
V = −14πε 0
Ze2
r= −
m2
Ze2
4πε 0
⎛
⎝ ⎜
⎞
⎠ ⎟
21n2
Total Energy
• The allowed values obtained by Bohr for the total energy (T+V) is thus
• The energy levels of the atom are an infinite number of discrete values
• Differences between two levels give allowed discrete jumps
• We get the gaps in the solar/atomic spectra!
€
En = −m22
Ze2
4πε 0
⎛
⎝ ⎜
⎞
⎠ ⎟
21n2
Planck’s Constant
• Planck's constant ħ= 2π x 6.626068 × 10-‐34 m2 kg / s
• Energy and angular frequency
• Angular momentum
€
E = ω
€
L = n
Another odd fact
• Double-‐slit experiment
Classical Par7cles Waves
Wave-‐par7cle duality
Electrons
Electrons behave as waves and par7cles
• While in transit electrons have wave-‐like proper7es
• Electrons have par7cle-‐like proper7es on detec7on
• Classical theory of waves, the intensity is the square amplitude of the wave
• If a wave has amplitude, Ψ, then intensity, P, is given by
• At posi7on (x,y,z) and 7me t
€
P x,y,z,t( ) = Ψ x,y,z,t( )2
Quantum Mechanical Wavefunc7on
• By analogy with classical waves introduce the wavefunc7on Ψ(x,y,z,t)
• Plays the role of a probability amplitude • The probability of finding a par7cle at posi7on (x,y,z) and 7me, t, is propor7onal to |Ψ|2
• In double-‐slit experiment, let ΨA be the wavefunc7on at a par7cular point on the screen corresponding to waves spreading from slit A
• Let ΨB be the same for slit B
Superposi7on
• If one slit only is open then
• If both slits are open then we get
• Note, the amplitudes are added, not the intensi7es (probabili7es)
€
PA = ΨA2
PB = ΨB2
€
Ψ =ΨA +ΨB
P ∝ ΨA +ΨB2
Linear combina7on
• Let
• Then
• where the 3rd term which gives the contribu7on to the interference
€
ΨA = ΨA eiαA
ΨB = ΨB eiαB
€
Ψ = cAΨA + cBΨB
Ψ2
= cAΨA2
+ cBΨB2
+ 2ℜ cAcB* ΨA ΨB e
i αA −αB( ){ }
Interpreta7on of result
• Emphasize, unlike classical waves Ψ(x,y,z,t) is an abstract quan7ty
• Its interpreta7on is sta7s7cal in nature • The intensity (square amplitude) is a probability func7on
• Therefore we can “normalise” Ψ such that
• We will return to this point soon
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Ψ*Ψdxdydz =1∫∫∫